no rear lights

[fixitguy1ca]

Hi,
I have a 2001 gmc 3500hd tow truck. I have no lights at the rear. If i jump 12v to the junction box under the seat everything works, as in 12v to the park lamps they work, 12 v to the left signal it works 12v to right signal it works etc. the park lights work on the cab, as do the marker lights on the roof. they all stopped working out of the blue. I replaced a faulty "multi function switch" but it just makes the hazzard lights work now. They work on the rear. There are no brake lights, tail or marker lights, or backup lights at the rear only. It is not a wiring problem at the rear as I stated earlier. The signal lights do not work either.(not on cab or rear). Is there a system common to these items? Is the 4 way flasher and turn signal flasher 1 and the same? If not where is the turn signal flasher located? I have checked the fuses and they seem to be all good. I am stumped(and I an a master electrician). I'm about ready to just rewire and relay the whole thing but before I do I figure I'd ask the experts here. Thank you for any help.

[DmaxMaverick]

Welcome aboard!

Is this in fact a 3500HD? If so, the 2001 is a GMT-400 body style (88-2000), which was carried over for a couple model years, including the 3500HD (I'll move it to the appropriate forum, if so). If it's a new(er) body style (GMT-800, 1999-2006), you have a 3500(non-HD). The electrical systems are very different between the two models. However, both have an underhood electrical center (above the driver side fender tub), which should contain the fuses for the body lighting (as well as trailering/aux lighting).

The turn signal and 4-way flasher should be one in the same, but may be different or unique on the 3500HD (many upfitter options). On the GMT-400 trucks, the hazard flasher switch (on top of the column) can be problematic, and they often fail, killing various lighting circuits, depending on model series. Occasionally, just cycling the switch repeatedly will clean it up and resume normal operation (for a while). If the switch doesn't "feel" right (hangs up, drags, noisy, very loose, etc.) when cycled, it's probably bad. Tow truck duty probably requires an above average use of this switch.

[fixitguy1ca]

Thanks for the reply. I fixed the problem tho. Yes the switch was bad, there was also a bad ground to the relay block that controled the deck. There was also a blown fuse in the drivers door fuse panel. The switch fixed the brake lights. The ground fixed the tail lights the fuse fixed the turn signals and backup lights. Makes me wonder what engineering genius designed this system???