i have a 2001 blazer and 1 head light is yellow and not bright i had same problem with a other car and it was ground dose any one know with color wire is ground or know what my problem might be

 

It might just be an older bulb than the other one, I'd just replace it and see if that fixes it.

 

If the bulbs are the same age or have been replaced and the problem still persists, it could be a wiring problem as you suspect. Assuming that you don't have the ZR2 offroad option, THIS REPLY in another thread should get you going on the wire colors, etc. Found it using "headlight+schematic" in the search function.