This is a 2004 Boxster S. Out of the blue my passenger tail light and side marker stopped working. I have had problems before with the brake light which usually was resolved by just pushing in the metal connectors on the tail light holder (common problem I guess it pulls away from the plastic case) but this is completely different. My brake light and turn signal work fine. Hooked up to the connector for the driver's side headlight all light function appropriately so it is no the bulbs or the metal strips pulling away. I check the A3 fuse which would control this and it is not blown. I even tested a good fuse in its slot just in case and still it does not work.

I can replicate the issue on the driver's side by pulling the A4 fuse, it only controls the tail light and side marker leaving the turn signal and brake light functioning, so I know that the problem is in the A3 circuit somewhere.... but how will I be able to find it without having to literally rip the entire car apart to get to every inch of the harness? I know that the headlight switch can go bad in these cars but everything else literally works flawlessly and the switch still feels good and "clicky". Is there a way to test it? Are there places to test that might be easier to get to? I swear If I can't track this down I'm just going to run wires from the driver's tail light and side marker over to the passenger side like the fog light hack.

Any information or wire hunting experience is appreciated

 

Also I want to add just for clarity that this is not the fog light. I know that the fog light only lights up on the driver's side on these cars. The problem here is with the tail light and side marker light.

 

Do you have a multimeter or a testlight? It helps a lot to know if power is getting to the plug & if the plug has a good ground. There's a bunch of ground wires on the bulkhead panel under the carpeting. On mine, the nut that grounds them to the chassis had backed out and gotten loose. In a humid climate, they could have corrosion, too. Clean all the plug contacts with contact cleaner and lightly grease with dielectric cream.

Don't you just love wiring problems?

 

So I did some testing on the A3 and A4 slots on the fuse block. With the key on and the switch off both do not have power. When I turn the headlight switch on A4 receives 12 volts and A3 still has nothing. So the fuse is not even getting power. Really the only thing between the battery and the fuse block is the headlight switch correct?

 

Are you checking before the fuse or after the fuse? Have you grounded your multimeter and probed the supposedly hot side lug of the fuse? Have you tried swapping fuses a4 to a3. does that make any difference. I suppose you could have a bad contact within the headlite switch. Next thing is to take the switch out, check for voltage on the wiring, and see if the switch lets continuity pass. Kinda surprising that the engineers spec'd two separate circuits for the marker and tail lights.

 

I think its more likely that the fuse is upstream from the lite switch. Key position shouldn't have any effect, the tail and markers should work even with the key switch off and the key removed.

 

The fuse is definitely downstream from the light switch or else it would not be able to modify whether or not the fuse received 12 V. Key position was only mentioned to illustrate that there is not supposed to be any power to those fuses at any time with the switch off. I jumpered the A3 and A4 fuses and everything works perfectly so this will be a temporary fix until I can source the actual issue which is probably the darn switch. Does anyone have a schematic for the poles on the back of the switch and which one takes in 12 V and which one distributes power to the tail lights?