don't have any tail lights and a 1990 GMC Sierra
Check both wires going to light sockets. Must have power on one and ground on the other.
Probably lost your ground circuit.
that will make all circuits hot
No for lights to work one wire has to have power and the other has to have ground. Without either your lights won't work. Need to find out which circuit you lost. Power or ground. You stated there was power so that leads me to believe the ground circuit is lost.
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