[atcos152018]

1987 S15
no taillights
when I turn on taillights, the gas gauge goes to empty.
When I turn on blinkers, the gas gauge goes to empty and both taillights and license plate light flash.
Any ideas?

[Mr. T]

Run a temporary ground from a KNOWN good ground (I use battery) and see if it suddenly improves. Frames and pickup beds all make bad connections after a while.

[PHWOARchild]

Bad bulbs will do weird things to electric circuits. Check those grounds too. Swap bulbs from side to side and see what happens, remove them one at a time and see what the gas gauge does. It could be a short in a wire under the dash, got critters in your garage?

[joe_padavano]

There is no factory connection between the gas gauge circuit and any of the light circuits. Change all the bulbs you want. That's not the cause. This is most likely a pinched wire harness somewhere between the dash and the back of the car. There's a short somewhere in the harness that is tying these circuits together.