I have a 2003 monte carlo and I recently noticed that my door panel lights only partially work and when I switch from low to high beams I sometimes lose all lights. How can I find out what the problem is without having to pay a garage?

 

Sounds like a bad/faulty Multi function switch/sensor.

Do you have brake lights?
Since the problem changes when you use high/low beams...
Im gonna say a Multi function switch.

A quick check...
Using the brakes and or turn signals...
Try jiggleing the turn signal and see if the tail lights flicker etc..
If they do...that's the problem.