Dim sidemarker, low beam headlamp and turn signal

Bulbs don't get dim. They work or they don't. What you're describing is typical of a broken ground wire. GM has had some really bizarre headlamp circuits, but in general, if you have a dim right low beam, it will go out completely if you unplug the LEFT headlamp bulb. That works on all other car brands. Due to their weird daytime headlamp circuit, that might not work on your car, but it's worth a try. Also, unplugging the right headlamp bulb might make the side marker light go out. Any observations like that point to a ground wire problem. The most common place to find the break is where all the ground wires for those bulbs are spliced together or are bolted to the body.