Inspired by this ye olde thread: https://www.starquestclub.com/forum/index.php?/forums/topic/37443-rewire-your-taillights-aussie-style-pics-video/
I don't know about you, but it's always annoyed me that the outer taillight bulbs in the USDM Starquests are just that; taillight bulbs. They take the same type of bulbs as the inner two, and the sockets are even keyed for one-way installation, so why the hell don't they function as the turn signal/hazard lights? Was it a market-specific styling decision? A regulation-compliance thing? Regardless, to me it was stupid and I decided to do something about it.
Basically, what we're going to be doing is moving the turn signal/hazard function of the inner two brake light bulbs to the outer bulb on each corner while retaining the brake light function for the inners. The AUS/JP harnesses may already have this wiring set up, but I'm going to guess most of us don't have those lying around. You're going to need:
Standard pliers AND channel locks
A soldering iron and solder
Wire cutters and wire strippers
A good roll of wire
Heat-sink butt connectors
A set of "helping hand" soldering wire clips
A small file
A small flat-head screw driver
Straight pick or terminal extractor/depinning tool (if your flathead screwdriver isn't small enough)
2x 1/4 inch female quick-disconnects, 18-22 gauge
2x 1/4 inch male blade connectors with locking tabs (I got mine using an extra taillight harness I had lying around)
and some patience. For the record, I'm working with an 87 Conquest.
The below steps mostly go over setting up the passenger stop lamp harness, but the same apply to the driver side.
First, disconnect and remove your stop lamp harness from your taillights. You'll most likely have to remove the whole taillight assembly to do this (2 nuts/studs near each taillight bulb) but once you get the harness out, you can reinstall the assembly; the harness for the outer bulbs should be long enough to reach around the bodywork.
Once you got the harness out, the only socket we're working with is the outer bulb socket (not to be confused with the reverse light socket, which has much shorter wiring). Looking at it through the bottom, you can see an empty contact pad. This is how we'll be providing power to the other filament for the turn signals/hazards.
We need to remove the metal socket from the plastic housing. Put one of your bulbs in the socket to prevent deformation. Then, use your channel locks to hold the plastic housing while you use your pliers to twist and pull the metal socket as pictured below. Don't squeeze to hard with either, and don't twist the metal socket too much; you could damage the ground strap which has a relief in the plastic housing. Just twist the socket back-and-forth carefully while pulling outwards, and it should come out.
Now that we got the socket out, we can start modifying it for our new circuit. At the bottom of the socket is the contact pad, which is held up with a spring and stopped using tabs/reliefs in the socket housing. Take your small screwdriver and push down on one of the tabs from the inside to to tilt the pad, so the OPPOSITE tab jumps out of it's relief hole in the socket
Once one of the tabs is free, use your small screwdriver to push out the pad:
The taillight (GW) wire should give you enough slack to work. Next, get your "helping hands" to hold the contact pad. Use your soldering iron and solder sucker to remove the solder from the (currently) unused pad connection until the hole in the pad is present, then feed your new wire through the hole and add new solder like so:
Take your wire cutters and clip as much wire/excess solder off as you can, then finish the job with your file using the other untouched pad for reference. It doesn't need to be perfect, just close enough; the spring in the socket should take care of any small differences in height:
If you haven't already, feed the new wire you just added through the center of the socket like the existing taillight wire is. If you want to be really neat, you can run the wire through the existing harness sheathing, but I'm lazy and my wires were too big, so I opted not to.
Now, reinstall the contact pad to the socket. It's crucial you keep in mind the orientation of the pad when you install it as the contacts on the pad correspond to the different-brightness filaments in the bulb. Luckily the contact pad itself has different sized tabs which have their correspondingly-sized relief holes in the socket:
Once it's installed, you're good to re-install the plastic housing by forcing the socket back in, just keep in mind the relief for the ground strap.
Now, so we retain the ability to remove this harness from the car without cutting wires, we're going to be adding this wire into the harness connector, which luckily has 2 free ports.
Take one of your blade connectors and crimp it on to the new wire you just soldered in. If you have an extra taillight harness like I did, you can get your blade connectors from there by depinning it like this, then wire it in to your new wire:
Once you get the blade connector on your new wire, let's get it into the connector. First, though, we want to reset the catch on the blade connector so it doesn't back out:
Remove the connector like so, then you should be able to feed the pin through one of the open slots in the connector:
We're finished with this harness. Go back and do the other side. Both should look something like this. If you didn't run your new wire through the sheathing, make sure it runs through the gap in the housing so it fits properly on the taillight housing:
Now we move to the back of the car with the harnesses. In order for the outer bulbs to act as the hazards/turn signals, we need to move a couple of terminals in the connectors on the car's end. At the taillight harness connectors, de-pin and move the white wire with the red stripe (or blue stripe, for the driver side) to the slot corresponding to the new pin you added at the tailights:
So, as it stands now, the outer bulb will be the only one acting as the brake/hazard/turn signal, so we need a new brake light power source for the inner bulbs. We can get this at the 3rd brake light, connector E-02 specifically. This connector is just prior to the driver-side hatch strut:
The wire we're looking for is green with white stripe. Tap 2 wires from this, one long enough to get to the passenger taillight and one long enough to get to the driver taillight. This is how I did mine, I challenge (i.e. highly suggest) you to do better, these wires are basically getting battery voltage when the brake switch is closed:
For those of you who know better, I did at least get some electrical tape on the exposed portions of wire.
With the wires hooked up, get your female quick disconnects crimped to the other ends. You can test fit them like so, they should be snug:
Now, in your taillight connector, fit your new wire in the same spot where the white+blue/red striped wire USED to be (top-center). Make sure it sits as far forward as the rest of the female connectors, and doesn't come out too easily:
Now you can get everything hooked up to the taillights. All 3 bulbs should be the brake lights, but only the outers will be the turn signals/hazards. This is the finished product:
If anybody's got suggestions for improvements to this, let me know.