I have a 1989 GMC Sierra that the headlight/parking light switch started smoking and now I have no dashboard lights, reverse lights, outside cab lights or tail lights. The headlights and brake lights work. The wiring harness to the back of the switch is black on the brown wire. Do I need to replace the switch or the whole wiring harness. I cannot find either part anywhere.
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You might want to start by checking the fuses associated with your headlights. If those are fine, check the headlight relay. You can do a search online to see where the relay is located on your vehicle.
The front park lights work correct? If the front park lights work than there is not a problem with the light switch since the brown wire from the switch supplies power to the front park lights, front side markers, and the rear taillights. There should be twelve volts on the brown wire from the headlight switch with the connector plugged in and the park lights turned on. If this is true that there is a problem with the wiring to the back of the vehicle. A thorough inspection of the harness should reveal the problem.
The dim break and blinkers support the above. High resistance also know as corrosion, in the wires or connections would cause this problem.
The fuse is a good start. The headlight may also have a relay, which may be in another box similar to the fuse box. Check the fuse before the bulbs - it's more likely that a single fuse would blow, than both headlights blowing at once. the switch. Not sure where this is located on your car (dashboard? blinker column?) but that's another single point of failure that could explain both lights malfunctioning at once. check the fuses for the headlights, and the headlamp relay first. good-day !