105 Tail lights

I hooked up my 105 to the M35 and started checking lights before i left. It's been a while since i needed the trailor. The brakelights don't work of the service drive is on. But they work fine if the swith is set to stoplights. The truck's lights work perfectly. I have installed a complete front and rear wiring harness on the truck since the last time i used the trailor. I'm going to hook the truck to a friends trailor this weekend to rule out the truck. All other lights work great, blackouts,turn signals, parking lights. Any ideas? Mabye a ground problem? Do the trailor lights ground through the pindle or the plug? Thanks.

Wire 90 on the truck side of the plug is the ground wire for the trailer harness. It is in position D. Which is on the bottom drivers side of plug standing at the truck tailgate and looking at the truck.

Thanks, Barman. Looks like i'll be spending some quility time with my meter.

No problem, I just happened to have a recepticle on my desk. by the way, wire 90 is only about 6" long. It should be looped back to the recepticle mounting screws for grounding or another bolt just for that purpose real close to the recepticle.