When my headlights are on my brake lights will not work?

Does the center high mount stop working as well? If it does then the most likely issue is the instrument panel module. The way we need to confirm this is test the power coming out of it from the stop lamp switch signal. Clearly you will have power with the headlights off but we need to know when they are on.

Here is a guide that will help with this:

https://www.2carpros.com/articles/how-to-check-wiring

I am pretty sure this is the issue because if the center high mount works with the headlights off and then not when they are on, the only common thing is the module. However, it is nice to test it and confirm this as there is a chance that the multifunction switch is the issue but it is unlikely.

Let me know what questions you have. Thanks

Okay. So if the center light works properly then it is the switch or the module. I would tend to agree that if the switch was just changed then it is most likely not it. However, it does make me nervous because new parts do fail. However, you can test the wiring as stated above or if you still have a warranty on the switch then you can try and get it exchanged. I would like it if we proved out the wiring but if you are not comfortable doing that then I understand. Otherwise, I would replace the module as shown above.

Let us know what questions you have. Thanks

So I think to test this we need to test voltage coming out of the fuse block or at the taillight assembly on the light blue wire. Test this with the head lights off and on. Clearly there should only be voltage there when you press the brakes.

You can you a test light for this as it would be easy to use this rather then the meter. Here is a guide that helps with this:

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

Also, did this just start one day or did you just notice it? Since the third brake light is not affected, I am wondering if there is just a wiring issue where someone wired the park lamps to the brake lamp circuit. So we need to make sure we have voltage on the stop lamp circuit when this happens.

Here is the wiring diagram for your review.