Ok, Using GM electronic service info, your truck uses standard old school manual switches without serial data. The switch default to ground when released, then applies B+ to the door lock actuator depending on polarity for up or down. If you have a good Digital Multi Meter (DMM), go the the actuator and up plug. Carefully insert meter less to not damage connectors, but turn dial to DC voltage (If equipped with Capture / Record, use that also). Have a buddy push the lock switch either way and measure voltage. Your reading will be less than battery due to all of the connector, fuses, switch, etc. It should read the same as fuse box voltage. The actuator is a series circuit, designed to consume all of the voltage, so if plugged in, back-probe the motor, hit the switch and reading is now voltage drop across the device and should read fuse box voltage. If low, Check B+ (1 wire is hot depending one lock or un-lock) still fuse box B+. Check using a known good ground like "E" brake bracket and other wire at the motor. A good reading should be less than 0.500 volts. This is voltage drop. If higher (Say 1.2 volts) this would indicate switch contacts, connector condition (Clean with electrical cleaner, add Dielectric grease), but B+ wire in door jamb boot, connector inside to allow the door to be removed and both side of the circuit breaker. Remember, to turn the circuit on to draw current during test...Other wise you will not know if the circuit can handle the average required to operate and make the actuator's motor strong enough to handle the normal force required to function. Some testing, then back tracking will solve mystery. Just remember, that terminals inside plastic connectors can burn, turn colors, become spread and corrode, thus dielectric grease. Help this helps, but this is what we did in the dealer. There are several good vid's on voltage drop, but also found in section 8 or electrical service book with pictures & explanation of what you are seeing. Hope this make sense. Best of luck, but best practices.