Just found this diagram for slightly older accords. May swap ignition relay for back up relay at fuse box area tomorrow and see if #21 fuse still blows.
Cannot locate any individual grounds in trunk area either. Where are they?

Here is what I have for the underdash fuse panel.

Here is a backup light circuit for your '04

Thanks for the diagrams Poorman!!
It looks like if I read it right, on my AT car, the relay is upstream and could not blow the #21 fuse. Also I gather I don't have a Back Up Light switch on the actual transmission in the engine bay - that's only for MT? So my sending unit is probably below that center console. But the AT shifts into all gears fine as long as fuse #21 is good, or I use manual shift lock override, so i don't think it's the transmission range switch. The gears were indicated fine on the gauge and on the shifter as it went into gear...

I thought I smelled a little burning smell the other day and saw a wisp of smoke very slight at night in the headlights coming in front of car, from roughly where the AT would be - this was during and after I had been using my 6-24 V test bulb to connect the #21 fuse and test the PW, Moon roof, blinkers etc... for several minutes. Could be nothing. But had me thinking it was some sort of Reverse switch wiring going up in smoke. Any thoughts on that? Is there anything like that out in front on the Transmission?

I may switch the IG relay and Back up relay mostly for my amusement today, but I think again, that it can't be blowing the fuse :(

The "reverse" switch is in trans range switch which is on the side of the trans.

Also found this in a Mitchell pdf shift interlock diagnostic:

Poorman you tha man! Thanks!. Going out with my ipad now to go mobile with your diagrams.

Just snapped pic of my owners manual sold with car. Look at #7 description. Honda idiots.

I just swapped the ignition relay with the reverse relay marked "ACC" in my car. Same thing. the Reverse relay will work and allow me to start car. (no relay in that spot I can't start car so that's how I know both are working).

I've been all over the engine bay from top and from the bottom as well. I cannot find any lengthwise front to back long connector on the side of my Auto Trans. I can see about 13 other connectors of course none of which I have any clue what they are.

Anyone had experience with this connector and can give a more descriptive way to find it? I assume that diagram is Down = front, Up = back of transmission from a bird's eye view looking straight down on the Auto Transmission.

Hell, what if I just pull the reverse relay altogether out? It IS listed as ACC so does that mean I lose a lot of stuff OR only just the reverse lights and the AT dash board indicator status of what gear car is in? According to the wiring diagram that might be it.
At least for now, I might live with that. It's been over two weeks here like this, and I think I also read having #21 fuse circuit down also means I have no radiator fan??? Is this true? If so I could overheat my car in CA summer heat - not good.