1994 sierra, how to check license plate lights

[PDF]Wiring Diagram.pdf - Ken Gilbert

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Each connector number shown in the wiring dia- ... Rear defogger cord (Ground). Fuel tank cord,. Roof cord, ... circuit. Then check the switch, relay, fuse, ground, etc. 4. .....Rear wiper motor. Rear wiper ... License plate light RH. Tail light LH.

#? license plate light - Circuit Wiring Diagrams

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#? license plate light - Circuit Wiring Diagrams - Page 3

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The license plate lights have their own separate bulbs. If you are able to replace the bulbs, that may solve your problem. If not, you may have a loose or rusted connection to the plate bulbs only. If you are able to gain access, you may want to check the condition of the harness that supplies the power to the license plate portion. Good luck and have a great day.

It's possible these are two separate problems. You will have to find out why the power is not reaching the license plate lamp assembly, and this means tracing the circuit. Check for power and ground at the lamp. You will have to identify the correct wires and follow them back toward the fuse to find the problem. A wiring diagram will make it much easier to solve.
Same with the trailer connector. If it's not working, I assume you mean there is no power when you plug in your trailer. You will have to figure out which wire is supposed to supply battery voltage and why it isn't. Are there any broken wires? Corrosion? Does the trailer connector have any connection to the license plate lamps? (It shouldn't.)

run a wire from the tail lights to the plate light . if this works the wire to the plate light is broken , if not replace the light socket

You didn't forget to pull the headlight switch to check them? That's odd, usually license plate lights are spliced in from the tail light circuit.
There may be a separate ground for the license lights-if so, should be right nearby the bulbs. Use a test light or voltmeter and make sure fuse 22 has power. You could also check the sockets for power. If you have power to the socket and new bulbs don't work, it would be a grounding problem.

There should be a pigtail connector for trucks with or without rear bumpers. Did you look in the harness for a brown wire ?
Same as the marker lights.

You shouldn't need the license plate lights to work to make your trailer lights work. Chances are that there is an issue with the wiring it's self. On my father 94 pick up the wiring is spliced directly in the taillight wiring. If your taillights work there shouldn't be an issue with the trailer lights. Start by checking or remounting the ground wire attached to the wiring harness. If that isn't the issue test the input of the harness followed by the output. If there is input but no output you need to replace the harness. Try the CURT 55379 you can purchase this by visiting one of their retailers by clicking here.

run a wire from side light connection to the no plate lamp

It sounds like either a bad headlight switch, or a poor ground, check the body ground and make sure that the tail lights are getting a good ground. Most likely it is the headlight switch.

Probably not... For some reason license plate light wiring and sockets are VERY prone to corrosion. Make sure to check the light sockets and try to get to the wiring close to them, your problem is more than likely very close to the lights.