Just replace the bulbs.. Cheap and if that does not fix it, start checking ground wires.. That's usually the next easy thing to fix..
I once had a trailer where the brake and turn signals did not work, on both sides...
But the running lights worked fine...
They are actually 2 different wires and elements inside the bulb. Turned out, it was just the bulbs. After about an hour of checking all the wires, fuses and such, finally just put a new bulb in and BINGO.. Worked fine.. Just happened that both bulbs went out.
Go figure..
Good luck!
Mitch
