Welcome to just answer.com. I'm here to help you and your 100% satisfaction is my goal. This can be caused by a number of things often a faulty are relay or faulty central time are module and occasionally corroded or damaged wire terminals at the module Pull out the park lamp relay located in the Junction block inside the vehicle, the relay should click when the removed or installed in the junction block. If not the park lamp relay is bad.
Check pin 85 in the Junction block of the park lamp relay for a ground with the park lamp switch off. If grounded check the Red/Yellow wire between the Junction block and Central time module at pin 4 of the C1 connector for a short to ground.
Check the powers, grounds and connections for corrosion at the CTM

Actually before we do that go to the headlight switch itself and tell me if you have power on the black and yellow wire coming from the headlight switch with lights turned on, this goes to a small junction block on the left side of the dash where it splits in different directions if we have power at the switch then checked the wires at the junction block and see if we have a bad connection there

If the parking lights work in the front, then go to the rear of the vehicle and locate the wiring harness where the trailer wiring splice is into the harness this is where everything separates and goes to the rear tail lights very commonly you'll find broken wires or corrosion in this connection

Okay now they go to that junction block and then split front and rear do you have power on all of the wires at the junction block in the left side of the dash?

Very good let me know what you find and we will test further if we need to thank you