Turn Signal Troubles

OK, try this. Remove both rear bulbs. leave everything else alone. Apply brakes. Does front light come on? If yes, Im stuck. The problem is in the harness. But If no, then one of the rear bulbs is feeding into the running lamp circuit due to; A) wrong bulb or bad bulb,or incorrectly indexed bulb, or bad socket. I am assuming you have, known to be good, correct bulbs and have correctly installed them;
- so To check the socket;, working at the back, key off, bulbs out,stare down into the sockets. You should see two little contacts sticking up off the insulator. When they overheat they sometimes melt down into the insulator and come into close proximity to eachother.Then the bulb could short the two circuits; brake and running.Its also possible for them to short underneath the insulator. Also that insulator is supposed to have a little tab on it which orients it to the socket. If its broke off or if for whatever reason not working, then the insulator can rotate relative to the bulb indexing pathways and again the bulb contacts can bridge the socket contacts.Just a thought.
-And finally,remember that the dual filament bulbs share an internal ground pathway. if the rear ground is not working it is possible for one of the filaments to attempt to ground through the other filament thus appearing as a crossfeed. I have seen this many times. I know others have mentioned it,but again PROVE your grounds. Grounds are always the first line of defence on vehicles that use the chassis as a ground.