Okay to answer your question maybe you partially tripped your crash sensor in ur trunk? I dunno if this is possible but take the molding out and see if you can push the little red button back in. Its on the drivers side next to the tail lights.
I know your all concentrating on the shutoff, but does anyone else think those heads are hurting him? The larger combustion chamber loses compression, and the ports and valves are much bigger than needed, so the a/f ratio loses its velocity. Plus, it all gets clogged back up if your still using origional 302 headers.
No codes came up when I checked for them. The cutt off switch in the trunk is fine, I checked it a few times just to make sure. Back to the heads...I don't know what car they came from, I bought the car as is. I have 1 5/8" shorty headers so there is no problem there. I don't think I have any problems on my exhaust side, though these heads might be hurting me with the stock intake and stock cam.