Taillights do not work when headlights are turned on

I would start with the tail light relay or combination switch. Here is a guide to help you get started with the tail light wiring diagrams so you can see how the system works:

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

and

https://www.2carpros.com/articles/no-tail-lights-exterior-running-lights-tail-lights-out

Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.