Info Provided by Rich @ http://www.intellexual.net/faq.html#001
Converting cornering lights into fog lights
This modification is specifically for the 1992-1994 SC coupes, which came with cornering lamps instead of foglamps. Foglamps were introduced on 1995 production cars. Since it is an installation procedure for a basic automotive relay, you can use the same instruction applied to other electrical circuits on the car (DRLs, extra foglights, etc).
Please do note that the 92-94 cornering lamp assembly is different from the 95-96 and 97-00 fog lamp assemblies. Though the 92-94 looks a lot like the 95-96, you can not interchange them between bumpers because of subtle shape and dimensional differences. The 92-94 lamps run on 15w bulbs whereas the 95+ run on 55w H3 bulbs so the illumination on the road is not the same. The 95+ fog lights are brighter, but the beam pattern is a bit wider on the 92-94 conversion.
First you'll need to buy a few items: a 4 or 5-pin 30-amp automotive relay, an inline fuse holder, a 30-amp fuse, about 20-ft 12-awg wire, a box of t-taps (5 is all you'll need if you don't mess up), roll of electrical tape, and good solid connectors: either use solder or buy male-female disconnects. All of this stuff is available at Radio Shack and should cost you no more than $10-15.
Now you need to get to know the relay. A relay is essentially a mechanical switching device that is activated by a low voltage input to activate a high voltage switch. In this application, your low current trigger will be tapped off the parking lights. Your high current output will be the power supplied to the auxiliary lights by the battery via the relay. The relay itself should be a 1-inch black plastic cube with 5 spades on one face, and some sort of mounting tab on another. The 5 spades are labelled something like 30, 85, 86, 87, 87a (at least these are Bosch's labelling).
- 30 is your input voltage for the switched device.
- 87a is an open circuit (no power).
- 87 is a closed circuit that supplies the high current to your auxiliary lights.
- 85 and 86 can be interchangeable, but they are the low voltage leads from your parking lights.
You can choose either headlights, city/parking lights, or sidemarker lights as your signal trigger. I used the city lights because they were easiest to access, the relay can be switched with an input signal as low as 150mV, and in case anything went wrong, no big deal.
Hopefully everyone here already knows that every DC electrical device requires two connections: a postive (live) and negative (ground). The darker/colored wire is always live (yes black is a color) and the lighter/white is always the ground wire. (in AC it's slightly different). I believe on our cars, the live is an off-red and the ground is white.
Basically, your first connection is going to be two t-taps off the both cornering lights' positive wires. Now connect the two t-taps you just made with two runs of 12-awg wire, and lead both wires back to the battery pan. Twist the two wires together into one single wire and connect them to the 87 spade.
The 87a spade will have no connection because it is the open switch.
Lexus uses a switched ground system which presents a problem because your cornering lights aren't always grounded. So to resolve this issue, go back to the cornering lights and t-tap both of the negative (ground) wires. Run two lengths of 12-awg wire from these t-taps---the same way you just did with the positive wire---to the negative post on the car battery. Your circuit is now properly grounded.
Next send a wire from the battery's negative post to the 85 spade.
Now make another t-tap off of the positive wire from your chosen signal trigger (in my case the city light), usually the closest one is the best. Once you've established this t-tap, run another piece of 12-awg wire from the t-tap to 86 spade.
Last is to establish the big power! You will connect the battery's 12v positive post to the 30 spade. BUT before you do that, use that inline fuse holder you bought. It will be a black, plastic, well... "fuse holder", with a looped wire running from one end to the other. Cut the loop into two bare ends. Connect one end to the 12v positive battery post, and connect the other end to the 30 spade on the relay. You may need to splice or crimp extra wire to make the run to the 30 space. It is critical that the fuse holder is very close to the battery. Like less than 12-inches of wire between them both. Make sure you weren't eager and stuck a fuse in that fuse-holder yet, because that would create a live circuit.
Right now the relay system is completely wired up, but there is no fuse in the holder. Therefore no current is running through it so it is still safe. At this point, you'll want to check all connections from and to the relay and once you've determined all is secured and well, insulate it! You want to make the relay water and weatherproof. I did so by wrapping the entire thing in 6 layers of electrical tape. Feel free to use more if you want. Then tuck or mount the relay somewhere safe (IE somwhere away from water exposure and engine heat). I put mine next to the battery pan which remains relatively dry and cool.
Now you can insert the 30-amp fuse you bought and test out the relay circuit. Turn the light wand in your car to activate the parking lights. Your cornering lights should turn on as well.