What you should find is the problem occurs with just the tail lights turned on. That is due to a bad ground wire. You can prove this by turning on just the tail / running lights, then the left signal, then remove the signal bulb from the other side. If the left bulb goes out when the right one is removed, the left one has a break in the ground wire. With the running lights off, current goes through the left signal bulb, can't find a path to ground to complete the circuit so it looks for an alternate path. That is to double-back through the tail light filament in the same bulb, then go over to the tail light on the other side, through it, then to ground. Since three filaments are in the circuit now, there's three times the resistance, which means one third the normal current flow, and that's not enough current to heat up the heating element inside the flasher assembly, so it either flashes real slowly or not at all.
To know if the problem is in front or in back, if the bad ground is for a front bulb, the turn indicator on the dash will light up dimly too.
November, 3, 2012 AT 12:24 AM
Thanks Guys. This pointed me in the right direction. I found a short in my trailer plug in.