Headlights Dim

You need to find the under hood fuse relay box and check the 15 amp fuse for the side that is bad. There is a fuse for the left headlight and one for the right headlight and when on goes bad that side will dim. Left is drivers and right is passenger side. Let me know what you find. Also leets check the system grounds. Here is a guide and the headlight wiring diagrams so you can see how it works.

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Check out the diagrams (Below). Please let us know what happens.

Yup. To verify a bad ground, remove the other bulb on the left side. If the dim one on the right side goes out, you'll find voltage on the ground wire at the socket. It's supposed to have 0 volts.

They normally have a ground on each side but GM has a very bizarre way of wiring their daytime head lights. I'm doing this from memory for all other cars.

When there's a break in the ground wire on the right side, current wants to go through the low beam filament but can't find a complete path to ground. It finds an alternate path by doubling back through the high beam filament, then over to the high beam filament on the left side, then through its ground wire. That's why I told you to remove the left bulb. That will remove that alternate ground path and the dim right bulb will go out.

You can troubleshoot the problem either way. With the left bulb removed, you will find 12 volts on all three wires wires on the right side. With the left bulb installed, you'll only have 12 volts on the low beam feed wire, (or just the high beam wire if that's what's turned on). The high beam and ground wires will have around 6 - 9 volts. If you can't tell which wire is the ground wire, remove the right bulb, then measure the three voltages. One wire will have 12 volts when the low beams are turned on. One will have 12 volts with the high beams turned on. The last one is the ground wire.

Please start a new question. Once my first reply was posted, this question went "off the list" and no one could see it except those of us in the conversation. That means none of the other experts are going to see your question or have the chance to reply. That does you a disservice because I don't know the answer other than I would follow the wiring harnesses to see if they lead to a ground.

When you type your question, be sure to include the symptoms or the reason you're looking for multiple grounds. Include details like what you've done already, and be sure to list the engine size and mileage.