just bought a 89 ext cab. has the same turn signal issue, left one dim in the cluster display and dims other lighting...that means its pulling high amps (current) from the circuit. this can happen from an increase in resistance in the circuit which can be caused by poor ground connections, failing components like flasher module, or old dried up wiring. your trailer box could be part of that issue. did you ever make any progress with this
??? i know its an old post but hey...