You should try reg 1157 and 1156 bulbs on the bad side. See if that fixes it. Check the ground wire at the front and rear light housings, and check for corrosion in the bulb sockets.
Also check the ground, socket and bulb on the taillight.
You didn't mention if you have a trailer harness installed, if you do, check where the wires were twisted, spliced or if you used scotch lock splice connectors to ensure continuity.
If it's plug and play, disconnect and check for corrosion, and reconnect.