If we could tell what is wrong over a computer there would be no need for mechanics. I CAN suggest the radio and dash lights typically are a branch circuit of the tail lights, and they have their own sub fuse. Those dead lights are a dandy clue but the problem won't be in those circuits. The tail light circuit is the one with the problem.
You have one major advantage in that the fuse blows right away. Intermittent problems in this type of circuit can be real frustrating to find. Start by looking for a chewed up trailer harness or damaged wires in the trunk, but unless you find something that's obviously wrong, don't start tugging on stuff hoping to make the problem go away. It could, but then there is no problem to find and it could come back later. You are lucky to have it constantly acting up and we want to keep it that way so you can troubleshoot it.
The best approach after performing the visual checks is to replace the blown fuse with a pair of spade terminals, then use a pair of small jumper wires to connect them to a 12 volt light bulb. A brake light bulb should work fine. When the circuit is turned on and the short is present the test bulb will be full brightness and hot so don't allow it to rest against plastic door panels or on carpeting. I use a long harness I made that I can hang over the rear view mirror. Now you can go around the car and unplug things and move harnesses around. When you do something to remove the short from the circuit, the bulb will get slightly dimmer. When using this trick in many other circuits, the bulb will go out or get very dim when the short is gone. In this circuit you have a lot of lights trying to draw current, and since all of that current has to come through the test bulb, that bulb will be fairly bright even when the circuit is otherwise working properly. You'll have to watch closely for slight changes in brightness. To make the changes in brightness of the test bulb easier to see, use a head light bulb instead of a brake light bulb. The brake light will only allow about one amp of current flow. The head light bulb will pass up to five amps. That is enough that when the short is gone, the tail lights will glow enough that you'll be able to see them if you look closely.
Wednesday, May 30th, 2012 AT 7:22 AM