Light wiring questions


I switched to Truck Lite LED lights on my 1975 GMC C3500, but it’s stock otherwise. The front side marker lights flash with the turn signal as they are meant to. The wiring schematic shows the single filament side marker light’s positive wire going back to the light switch, and the negative wire going to the turn signal wire. Now, the back side marker light has the positive wire going to the light switch, and the negative wire going to the ground in the schematic. The rear side marker does not blink with the turn signal. If I connect my test light from the wire to the side marker light, and to the turn signal on the rear it blinks. Now, for the question, will connecting the rear side marker the same as the front side marker, so that the rear side marker will blink with the turn signal, cause an overload on the light switch, or the turn signal switch, or anything that I may have overlooked? Also, my utility trailer has Truck Lite LED’s. Can I wire the side marker lights to blink, too in the same manner?


What you want to do should be ok.

One thing though. You may want to get a heavy duty flasher for the extra lights.

After all the lights are hooked up, operate them and test the load on the battery and alternator.


Led’s generally have very little current demand. I would be very careful about changing the grounds around to achieve a lighting effect…Sometimes this can lead you into an electrical black hole…Running a ground back to the flasher doesn’t sound right. The flasher will not provide a ground for what was a running light…


This could be tricky. The way those side marker lights can act as blinkers is by letting them ground backward through another bulb’s filament. Then, when that bulb blinks, it causes the side marker to blink. The problem with LEDs is they are diodes. Diodes are one-way devices, unlike a regular filament type of bulb so the backward grounding through another bulb won’t work right when you swap in LEDs. Without seeing the schematic you are describing I hesitate to suggest how to fix this. It can be done, I’m sure, but I really need to see the diagram to comment further.


Hello, The front side markers are already working with Jamstraight (sp) LED with the + side to the light switch, and the - side to the turn signal wire, and that is factory wiring. It works as the factory intended. Part of my question was aswered by Department of Transportation DOT I can only blink amber side markers, so if I want to blink rear side markers I have to add amber. The red side markers cannot blink, they are intended to delineate the extremity of the vehicle. My other question was answered by DOT, too. I can blink amber side markers, as turn signals, on the trailer, with the same regulations as on the towing vehicle. So, what it boils down to; if I don’t want the trailer side marker to work as a stop light, too, I want to run the front turn signal wires back through some of the unused prongs in the 7 way trailer wire connecter, to ground the trailer amber marker lights. Otherwise I can ground them through the rear turn signal, which I already tested as working I just didn’t know if it had a long term effect on the turn signal switch as it is not wired from the front turn signals.