Ground probably. To test: remove lens, Turn on a blinker, touch a pointed metal tool with a wire in it to where bulb enters the socket. Touch both bulb case and socket at the same time. touch other end of wire to a GOOD CLEAN SHINNY ALL METAL place. When I say "touch" I mean dig in. Build this test rig from what ever you have and make sure the wire is attached well to the touch tool. Do this until it lights and blinks bright. If you get it to blink bright then break it down to what it needs I.E. separate ground wire from socket or bezel to a spot on frame. If you don't get bright blink: Another good test: take out the bulb you have trying to flash and, with one of those lighted testers with ice pick looking tip, attach the alligator clip to a good shinny ground and touch the tip end to a contact down inside empty socket, One of those is the blinker. It should flash the test light. If it does, proceed as above to buzz out the circuit including GROUND. The second test will tell you if you are getting good blinking voltage to the socket contact. Be sure the filament trying to flash is the bigger one, that one is for brake and turn, the smaller is for tail and park. When you say they work except when tail lights are on, it's either cross wired or the tail light together with flasher (both filaments at once) is "breaking down" a weak ground. Don't forget all this stuff goes thru the turn sig wiring and back out and thru various connectors. Check that stuff too. Report findings so we can all learn, that's what we expect here. You need to let us know. Maybe I'm on the wrong track but I need to know to continue till it's fixed. Others will have more ideas.
