I have a similar problem, my brake lights are working good but the park light not coming on, I tried changing the socket, the bulb but the park light still won't come on. The problem only exist on the left tail light though, any suggestions?
Some of them use individual grounds per bulb and others use one ground. If it's one ground well then irs not that. Get a tester and see the problem. Test light if no access to a dvom