Tail light ground

If the brake pedal was unhooked and dropped, then the brake switch would have been activated.
In all likelyhood the other side bulbs were already previously blown.

The bulbs ground to the housing which grounds to the body which is a huge chunk of unibody. Then under the hood there should be a short jumper-wire from the body back to the battery neg. the jumper is probably less than 16 inches long, and IIRC screws on to the rad core support near to the rad.
There is usually another jumper from the back of the passenger-side head to the firewall, and of course the big-boy from the engine block back to the battery.