Pontiac Repair: 99 Montana turn signal, pontiac montana, rear lamps

Question
Hi Todd.  I have a 1999 Pontiac Montana.  When I turn on the right turn signal, the left rear and left front side marker also flashes.  All of the right lights function normally.  If I then hit the brake peddle, the rear signal lights stop flashing and are on steady.  The front side marker then goes off.  The same thing happens when I turn on the left signal (right rear and side marker flash)  If no signal is used and the brakes are applied, then the rear turn signal lights also come on along with the brake lights.  If I turn on the 4 ways all lights function normally.  The van also has a towing kit (hitch and lighting adapter) installed but I'm not sure if it was dealer or aftermarket.  Thank you

Answer
Hi Ron:

I think the lats part of the question is the key to the answer. Thes vans didn't come from the factory with a towing kit. It's aftermarket. I would look at the wiring harness for the hitch where it's tied into the van's wiring or at the end of the harness where it would plug into a trailer. If you live it the rust belt like I do what happens with them it the end corrodes and the corrsion actually crossed the circuits so when one lamp is tuned on such as a parking lamp the turn signal bulbs will turn on also doe to the "short" circuit across the corrosion at the harness. I have also see trailer hits wirning have a "module" ties into the car/van harness and that go bad and suas the same issue.. The last think that is pssable is thes van had issues with the tail lamp circuit boards. take the rear lamps out and unplug the harness from the circuit board on the lamp and look at the connector for any signs of overheating if the plug or board is melted replace both parts. Look on both sides of the van I have see ones come in where both sides are damaged. Good luck :)