loud bang, bright flash, then the stop lamp came on...

That's the same for my 650. Good luck.

Ok, I know I haven't updated in a while, but I have been performing a bunch of interval maintenance that was well overdue first. I think I may have found the problem with the electrics, lets see what your opinions are. Since what happened seemed to be a brief and intermittent failure of the electrics, so I checked all the fuses wires etc. and found nothing. What I noticed was the the battery fluid was very low (well below the minimum line), I'm wondering if this could cause the electrical problems similar the the idea posed that the seat was crossing the battery terminal.

stop light is there to signal when the tail light bulb burns out.it lights up when the taillight lights up

it comes on when the circut is activated.
try taking out the tail bulb and see what happens. you'll get the picture.

I think you have a bare power wire under the rear fender grounding out start your checking there

well here is my thinking, what happened was a loss of ignition, dimming of the headlight and the flashing on of the stop lamp. Now if the circuit is designed to detect a burned bulb then a significant power loss to the circuit may trigger the same light.

I'm not so much worried about the stop light being on as the ignition failure. Which means checking all the ignition system wiring and looking for a possible short, which I did.

My valve cover gasket should have finally come in which means I can hook everything up and test some things.

Sounds like you have a short to frame in the lighting circuit, perhaps a weak or corroded battery connector. If you're really lucky you'll find some creative harness work from a PO.

I highly recommend going through the entire harness with some di electric grease.

Good luck.