This mod covers installation of fog lights in an '02/'03 Subaru Impreza TS. It would also apply to an '02/'03 RS.
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Here's the space you have to work with. I've shown the approximate depths of the fog light recess at the four corners, the hole diameter, and the spacing of the stock mounting bolts. These are again approximate numbers, so don't fabricate anything based on these. They should be accurate enough to determine if your light will fit if you choose a different model of fog or driving light. Note that there is a metal "backing plate" secured by the three bolts, which I feel is an acceptable mounting point for these lights. You can get aftermarket brackets, make your own, or just drill like I did. My method is the cheapest and suitable for my needs, although it is difficult to make fine adjustments to the alignment of the lights.
These are the lights I used, Hella 500 Fogs. The 500 Driving lights are similar but have a different light pattern. The fogs really light up the area to the sides and directly ahead of the car. They have a very wide beam and are quite bright. If you install these or any other auxilliary lighting, make sure to exercise common sense in their use. Don't blind or annoy other drivers, you're not the only one on the road.
* Fog lights (I used Hella 500's)
1) First we'll put the switch and relay in. Remove the negative terminal from your battery so you don't short anything out.
3) Remove the plastic clip (on the side) and two screws (at the bottom left and right corners) from the dash. The lower right screw position is not shown in this picture, it's in the next one.
4) Pull the dash panel towards you and down. There's some clips holding it at the top. On the right of the picture you can see the screw from the previous step.
In my car, the wiring harness and cable for the fog light switch were clipped into the switch blank next to the cruise control switch.
5) Put the relay in the fourth relay spot from the top.
6) Move the cruise control switch to the second switch position, then insert the fog switch into the first position. Insert the wiring harness for the fog light into the back of the fog light switch. It's the one you pulled out of the switch blank that was in the second position originally.
7) Replace the dash panel by clipping it back in and putting the screws back in.
8) Remove the fog light cover.
9) Verify that you have power to the stock fog light harness. Hook the battery up, turn the key on, turn the lights on, push the fog switch so it's illuminated, and check for 12V at the plug.
1) After you verify that you have power to the plugs, it's time to remove them. Unhook the battery again.
Cut the stock plugs from the wires. You'll have to cut some tape to free up some slack in the wires. I left 1" leads on the OEM plugs so that I could use them again someday if I wanted to. Alternately, you could find the plug that fits into this, but I just spliced with spade connectors so that the fog lights could be removed.
2) Loosen the plastic undershields (splashguards?) from under the area behind the fog lights. You'll need to take three of the clips out, a couple are under the shield, and one is at the front of the fender. The fender clip unscrews with a phillips and then pops out, the ones under the car unscrew by hand and pull out.
3) Move the plastic piece down and out of the way. It should swing down and allow you to access the back of the fog light mounting location.
I did this at normal room temperature. You probably don't want to bend the plastic if it's really cold. Don't force anything, it should be easy to move this piece down.
4) Attach the mounting bracket to the lights with the 10mm stud sticking straight out the back of the light. This will allow vertical mounting of the lights. Test fit the light, and figure out where you're going to drill the mounting hole. This shows the approximate location. Note that it's not necessarily dead center.
5) Hook the wires up and install the lights. I put a 2" fender washer behind the backing plate, and tightened it down. You'll have to adjust the beam of the lights following the instructions included with the kit. Basically that involves loosening the clamp that the mounting stud attaches to, and angling the light correctly.
I left the stock fog cover bracket in place. This allows the cover to be installed without removing the fogs, but don't turn them on if you have the covers in place, unless you want to melt the plastic.