This is a very odd one and I'm at my wits end....almost. So however this ends up PLEASE let us know how it ends up.

So here I go with some "wild" ideas.

Can you start the car, set the e-brake. Use the shift lock release and try to shift the car thru R to D4?

Next, that fuse also goes to the comb light switch......start the car and run the headlights thru all of the functions - park, headlight, high beams, left right turn signals?

 

I will absolutely post the fix to whatever this is.
Today I've resorted all day to manually unlocking transmission (even easier now with center armrest console gutted), and shifting in neutral with parking brake cranked on. I then put in fuse #21, THEN I start the car, and remember to never shift into reverse (or through to park either), before I pull over and stop, crank E Brake, put in Neutral, and pull fuse again.

I can easily shift into any gear manually releasing shift lock - or with a working #21 fuse provided I don't touch reverse. I can shift car off, or car running. All gears are easy to go into, and correct and show up on shifter and dash (if I do the above tricks) as correct numbers.
I do have a small bit of concern about the Transmission Range Switch graph I metered and made compared to the 2002 version - it's quite different.
I also noticed today that twice using my trick above to have gauges and most importantly P. Windows (!) working that the passenger airbag light came on and went off at a random time driving - I know that indicator is in circuit #21 but at that time, the circuit was alive and working fine (had been avoiding reverse with new fuse in). There was also a passenger with me in the seat. My previously seen CEL is nowhere to be seen today.

With or without fuse #21 circuit ok or blown, I can cycle headlights through everything just fine, low beams, high beams and fog lights all work. Hazards all work too. But if fuse #21 is blown, then my blinkers at that point won't work at all, but headlights still works fine in hi / low etc...

I guess since I keep blowing the fuse if I have a working fuse in slot #21, it means I either have a shorted component (and I think I've pulled them all and verified they all work on the reverse circuit except for the MICU), or more likely a shorted 12V live wire, not a bad ground.
Have not metered from Transmission Range Switch connector harness to the fusebox, nor from the fusebox to the Shift Lock Solenoid or the Pin Lock Solenoid connectors, nor from the fusebox to the rear seat driver side shoulder area connector yet. I guess that's all that can be done next or throw money at a MICU which means replace entire fusebox ?
?
Thanks for your continued help and ideas!

 

Unplug the transmission range switch and use a piece of wire to jump the white wire to the black wire. Turn the key to the II position (or what you normally do to blow that fuse) and see if the #21 fuse blows.

That yellow wire that causes the fuse to blow goes into the gauge control module, which is part of the instrument cluster. You may want to inspect the circuit board of the cluster to see if there are signs of damage.

 

Sadly it's goin to be at least ten days before I am back home to car now on a flight to Alabama!
I will try the yellow lines and such and disconnecting the Trans switch and post back what I find next.

 

Back at car. I feel like I am about to kick this car off a cliff or f-ing rewire the whole damn car myself.
Needless to say, nothing I try is working.
I put in a replacement Transmission Range Switch today and it instantly blew fuse #21 in reverse again.
I can ONLY get the car now to actually go into reverse and not blow the fuse in one way:
I disconnected a major wire connector under I think it's the TPS in the engine bay. Its probably a 20 or 24 pin connector and i could snap a pic but my gf took the crippled car. This is a four connectors plugin area about 3/4 to the back of engine bay up relatively high behind the motor. It was the one carrying the green wire with grey dots painted on it that eventually makes its way after several split offs to the transmission range switch.

By the way, I've tested for continuity and got it on every wire from the transmission range switch connector and up to the connector that disconnects a ways above the transmission.

I've disconnected so many things in the car - everything in the back trunk area at the four way connections at the rear seat drive side, all connectors in the shifter area (shift lock solenoid, cigarette lighter, heated seats, pin lock solenoid, etc...), disconnected connections around the center console and even the green wire connection at the ECM or ECT there and nothing will prevent blowing fuse #21 when I shift into reverse except for the above mentioned engine bay connector. I was excited I thought that almost certainly meant the Trans Range Switch was bad (shorting itself) and when replaced it, it did nothing to fix.
Visually all engine wiring looks fine. Nothing has been modified or replaced in there except for dealer service over 1.5 years back.
I tested the green wiring in the trunk area for continuity end to end and got continuity there.
I don't know where wires connect to where in the engine bay or what this could or could not mean. I don't even know if a wire will show Zero resistance (continuity) if it's shorted. I would think not, right?

 

Disconnected transmission range switch and jumpered black and white wires and put key in "II" position and fuse blew again. This was as expected. I am sure switch is fine.
I found some good wiring diagrams today but not quite sure how to interpret them, there are tons of diagrams and still not sure where to look next.

 

As pictured below - this is currently the ONLY way I can put car in reverse with AT shifter and not blow fuse #21. I have to disconnect this connector - the one pointed to - it's the upper connector of the two there.
It may be meaningless or insignificant. (?)

 

Does that fuse also feed the back-up lights? That looks like the trans position sensor, so how about a short in the circuit for the backup lights? (Its a long thread so maybe you've already ruled that out)

 

It most likely is a short in the circuit. That connector looks to be the junction connector C201 (a 33-P connector). The short may between that harness connector and connector C152. Check the Wht or Wht/Grn wire along that circuit.

 

Thanks for the responses!
Yes Jim, it is on that circuit for back up light (fuse #21), the Trans Switch wires up to it.
So there is some sort of a short somewhere.
I'm going to look at the pics and info in post 2 now.

edit:
So Connector "C 152" is the connector I've removed 2x and at the switch. That cluster of wires has continuity on each wire from C 152 end to the Trans Range Sw end.
(not sure if there can still be a problem. but they all have continuity, and looked fine. Connector pins all look normal / clean no black etc...). Trans Range Sw. is good. And, yes, it was C 201 I disconnected and then I can shift car trans in reverse without blowing the fuse, that's the only way I've found. So should I examine (how?) the wires from C 152 to C 201? And / or from C 201 on? Or, wait, I guess it can't be beyond that right... or can it, if the 12V gets returned from the Trans Range Sw back to indicate it's in reverse and activate back up lights, could it then be anywhere else upstream too? (?)

edit the edit:
Ok, RedBull, much thanks, after re-reading and studying all your attached pics, I think you're right, look between C 152 and C 201 for the short by looking at White/Green wire. Soo... should I pull apart the wire protector or try to run voltage up that wire - disconnected at both ends with a 12VDC test lamp in between that and the positive on my small extra 12VDC battery and the negative terminal? Is that a way to do this?

I have a small 12V DC battery for running a light. Should I shoot some of that up a wire to try to see if there's a short? Is checking for continuity enough to verify a wire is solid and not shorting? I'm a mostly new to this part, and online posts usually gloss over those details of how to actually check for shorts besides disconnecting things and narrowing it down.