I tracked it back after disconnecting all kinds of things, to isolate, and the only source for making the short to ground is a small black plastic wrapped wire(s) going up from above fusebox area to the A Pillar driver side.
The SRS airbags are there, so I am afraid to pull out all the stuff there.
What on earth goes up there?
Car has moonroof. could it be something that connects on that circuit only when reverse completes the green wire circuit??

1st pic has pencil pointing to 'bad' wire. this is only possible cause of getting shorting to ground readings. This continues on up the 'A' pillar driver side.

2nd pic is farther back showing left of steering wheel area, where the moon roof switch panel has been removed to trace wiring behind it.

 

In your second picture, I see a ground wire added for some aftermarket electronic accessory? What is that connected to? Many times aftermarket electronics are the cause of problems.

 

In your first picture (post #51), next to that black wire, the LT Grn 28 cavity connector is junction connector C405 and the LT Blu 28 cavity connector is junction connector C406. If your car was a manual transmission, then part of the circuit for the back-up light switch from C752 runs through C406; however, your car is an automatic; so, it doesn’t run through there.

For your automatic, there's actually a couple more junction and splice connectors in the circuit for the back-up light switch. For the automatic, junction connectors C501 and C601 (For your EX) are also in the circuit.

I haven't found what that black wire/cable is yet; but, I'm not sure if that would cause the problem for the No. 21 fuse blowing.

 

It does seem a little unlikely that it's causing all this, however, with that wire connected to whatever it's connected to, it is measuring a short to ground on the green wire, and if all connections are connected, that causes the P5 green wire from the MICU to the C Pillar C752 to also register a short to ground reading.
I ALSO discovered a very odd 2 Amp fuse right up in there, it looks like a 3 wire termination point, but in fact it's black / yellow 3 way tie in which I assume is either hot 12 VDC or switched 12 VDC. I don't know if it's related at all, but that fuse is toasted shattered inside.

I've pulled apart nearly everything in that region, C501, C502, C601 etc etc... (C405 and C406 aren't involved in the short to ground matter so they were left as is).
the only thing that has no connector to disconnect that's still in line with the short is that wire going up the A Pillar. I have a very short multi wire 12" connector with two white ends disconnected at Both ends (so it's like a floating wire), and both green wires register short to ground, (and also still the green wire on fog lights switch connector on the dash ALL reading short to ground). At that point on the two white connector cable that's completely disconnected, the only thing left I haven't disconnected to this floating unhooked wire is a spliced in T joint black wire casing - that's the one in my picture that goes up the A pillar. That's all that's left down below to give the short to ground reading.

I think that's not a wire going to ground. I'll go check. But this car is completely stock. Only factory (or dealer) supplied options. The car was bought new by the family and never altered.

 

If you can, try to take some more pictures of where that black/wire cable may comes from or goes to; so, I can try to find what is may be related to. Just seeing part of the wire makes it hard to search for the circuit.

Also, try to take some pictures of that 2 amp fuse, related wiring, and 3-way termination point.

 

So far, the only wire harness I see that goes up the driver's side A-pillar is for the driver's vanity mirror light, ceiling lights, ambient light, and moonroof. However, the only thing I see on that roof harness that might have anything to do with the back-up light circuit would be at the C601 junction connector, which you said you unplugged.

Junction connector C601 is related to the moonroof circuit for the EX and EX-L trim levels.

 

Ah wise you are RedBull. I thought it was all stock. I was wrong, and you're right, it IS some aftermarket thingee.
There is a multipin computer style black connector on one side. it looks like a computer charger or weird XL iphone kind of charger.
Also there seems to be a black sensor that was glued behind the dash on the right of the steering wheel area and directly in front of that was a piece of velcro adhered on (which goes to nothing we have).
I am not quite sure what it does, but it connects into some very large gauge wires that are crimped and barrel crimped off things in the fusebox area. If I disconnect this setup the car won't start.
No idea what it is, but calls will be made to track it down. I asked main owner of the car for past 6 years and was told that sometime they 'think something went wrong with the power door lock / key fob' - they 'think' but don't really remember.

The first pic is the 'sensor' glued behind dash. You can tell by slipshod way it was wrapped around things, this was NOT Honda work.
Other pics are the other side plug in that goes to nothing. and also the power source with heavy amp fuse I think.
I guess I thought at first this was the diagnostic plugin, but I found that farther up.

And, um, feeling a bit sheepish, I'd better go and double check I pulled that C601 connector.

 

This is a very long thread, and I just tried to re-read it. I have a simple question. What number fuse is blowing according to this picture you attached earlier?

 

The fuse #21 "IG Meter" is blowing anytime I shift into or past 'Reverse' on AT. All components in that circuit I've tested are fine.
Have short to ground on it, on the green wire.

RedBull: Just pulled C601 which in my 2004 EX AT is a 7 pin connector, yes 7 wire. And that made no difference. The Green wire is still reading continuity to ground with that disconnected.

 

I just discovered with all the fuses back in (and not in reverse) the mirror on the car is dead. There are two black buttons on it, a sensor and three wires in and out of roof. Moonroof has full functionality as does interior map lights both manually and on door setting.
Could this be the cause? That mirror must be powered up the A pillar enclosed wire case, the one that seems to be making my meter read continuity to ground on the green (reverse) wire.

I also see on wiring diagram that there are two other things spliced into reverse circuit: "navigation unit" and C601 + C602 "option connector". I don't know what these do. Is there a in mirror compass that determines position for nav unit??
I have the EX model. Not sure what options were on it. I've never used a nav screen on the 6 disc changer.