Remove the bulbs from the back-up lamps. Now you can check continuity to ground on the (+) wires for those lamps. With the bulbs removed, there should NOT be any continuity to ground from the hot wires.

I guess that would be the wire on terminals P5 on the 2nd picture that RedBull posted. If the P5 wire is worn through & touching bare metal, that would blow fuses.

 

I am trying to figure this out. I am using a 6-24 volt testor with a small bulb in it.
When I attach the alligator clip to the lower side of the fuse #21 connector and the pointy lead to the upper side the light is always on, although dimly, meaning I have a partial short - I think, right?
When I put car in Reverse, the bulb glows brighter, meaning that's what would cause the fuse to blow right there. But should I not be seeing a constant dim light? I tried another (working) fuse and it did not light at all.

Jim - I will try the continuity to ground test with bulbs removed. Although I have disconnected the Green wire that power the reverse lamps at the rear seat driver side shoulder area behind the seat and trim there, and still have the same fuse blowing in reverse.

 

Good. Since you've disconnected that and the fuse still blows, that means the short is "upstream" of where you disconnected it. I guess you unplugged it at C752 in RedBull's diagram?

Can you disconnect the wire up at the P5 terminal at the MICU? It's beginning to act like some part of that wire (from P5 to C752) is rubbed through & touching metal.

 

It does sound like he unplugged C752, located behind the left C pillar.

I am still wondering what all is plugged in or not, when he said the only way he can put it in reverse without blowing Fuse 21 is by disconnecting the C201 connector.

 

Ah yes, it's certainly C752 at c pillar I disconnected and fuse still blows in reverse.
And it's only unplugging C201 that makes the fuse stop blowing in reverse (with everything else back plugged in).
But is that simply cutting off the "return" current of 12V from the Trans Range Sw completing the reverse circuit and therefore interrupting sending it further upstream?
in other words would I check wiring between C201 and C152 or
Check P5 wire from MICU to C752? Thanks!
To say I'm a newb to electrical theory isn't totally off base here.

 

From C752 at the C-pillar, check the wire that goes forwards to P5. Check whether that wire has any continuity to ground.

What I'm thinking is that you might have a short-to-ground in the wire between P5 & C752. It doesn't blow the fuse until the controller switches the backup-lights ON.

Then similarly, check whether the wire from C201 to C152 has continuity to ground. But I suspect a short in that wire would blow the fuse 24/7 - not just when you shift into reverse.

 

Ah interesting Jim, thanks.
That does sound good theoretically to me since you've explained it that way. And it also seems there would be chance to short P5 to C752 anyway with the car frame right there and possible wear points in the plastics over the cable harness in the side floor panels.
I will check that pronto again. I've now wound up yet on the opposite side of the country from the car again on a family vacation.

And yes perhaps the engine bay wiring side, being shorted there, would short the fuse always with car in any key position all the time. And the voltage sounds like it must travel up from the trans switch back to MICU and down the line to short out when in reverse.

 

(Ok, flew back to area and Ohm-ed car now.)
With C752 C pillar connector disconnected and a 19 pin connector at the MICU - the vertical green connector just behind the inside kickpanel main fusebox - I Ohmed and grounded to a good body ground and DID GET CONTINUITY to GROUND on the green wire - tested both ends of green wire to be sure and tested about 9 other wires while in there.
The odd thing is I am also got a continuity to ground on a purple and black wire there - hmmm... maybe that's in fact the ground wire?... now I feel a little silly about that, I was suspicious... Don't have complete wire pin diagram for MICU.

But it looks like the tentative diagnosis is:
I have a short to ground from MICU Pin 5 (green with grey dots wire) somewhere running up to the rear C pillar driver side C752 connector. I will post my final results and fix as I find it.
Thanks to Jim, RedBull and Poorman for all the help so far!!!!!

 

Nope. That's not it. I have a short on this circuit, but it's not anyway up to the C Pillar C752. There's multiple connectors up over the fusebox that are not listed on any wiring diagram. There's some secret 3 way split on the green wire between about a 12" stretch of two large white connectors, and it's shorting to ground up there somewhere I cannot see.
there is a black plastic small wire going out of area to top left of the dashboard area - perhaps towards engine bay / firewall, and that must be the grounding to short cause in there or beyond.
If I disconnect all these I only short to ground up there, not at the back of C752 anymore.
so annoying this issue that goes on and on.

 

Can you post a few pictures of the driver's under-dash fuse box area;so, I can get some bearing of what you're describing and possibly try to find more info.?

Also, take a picture of the wiring connectors near the fuse box, to the left of the Data Link Connector (aka OBD II port).