We just started to install fog lights on a '05 LS Trailblazer. The LS edition did not come with the fog lights, instead these locations on the bumpers were covered by a pseudo grill.

Here is a preview of the final result:

As featured on TrailVoy.com here.

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Removing the grille

Removing the headlights

Removing the bumper

You'll have to remove all of this:

When I removed the bumper I was a bit disappointed. The car is relatively new and the metal bumper under the fascia had already started to rust:

Another view:

Repainting the bumper would have been a bit pointless, since the rust was being caused by rubbing between the metal bumper and the fascia, so we put some heavy duty duct tape on it instead, after touching up the rust.

This is what caused the rust:

Anyway, once the bumper is removed you'll have to cut out the following pseudo grilles:

Here is how the fog light would fit on it, showing how much we have to cut out. Please note that if you do not cut enough you'll get rubbing between the bumper and the fog lights since the fog lights will be mounted to the frame and not the fascia.

And the hole cut out with a dremel:

This is how the bracket fits on the car:

On the inside, where the bracket touches the frame, there are already two holes:

We can use this bottom hole for our bracket. The metal bumper on the other side of the bracket does not have any holes yet and you have to drill 2 or 3 holes in there. We used 2 clamps to hold the fog light and bracket in place while test fitting the bumper back on:

It fit in perfectly, so we drilled in 2 holes through the bracket and bumper and put in rivets. And on the inside where the bracket touches the frame we drilled 1 hole corresponding to the lower hole on the frame and held it in place with a bolt . We used stainless steel, so the bolt is quite a bit stronger than a steel bolt and thus allows you to tighten it a bit more and prevents it from rusting.

And here is a picture of the 2 holes through the bumper and bracket with the rivets:

Because we only riveted the bracket to the bumper, we can still remove the metal bumper without breaking the bracket by simply removing the 1 small bolt.

A picture of the fog light installed on both sides:

Then you reinstall the bumper:

Reinstall the lights:

And finally put the grille back on:

And that's how to install the fog lights behind the bumper.

The first option is definitely the easiest. All you have to do is add a T-tap to the correct wire, in our case we connected the fog light control to the (+) low beam. Then connect this T-tap to the coil on the relay and ground the other side. At this point you would need power for the fog lights which will be switched by the relay that was just described. The fuse box under the hood has a simple spot where you can simply add a ring with a wire to connect to battery (+). In this wire I suggest you add fuses. We used a 20amp fuse. Now the second wire you need is ground. You could connect this to the frame or the battery itself. Make sure that you can get a good contact and always test the ground after you install it with a multi-meter. This method allows you to have the fog lights automatically come on with the low beams

The second option is a bit harder, however this would allow you to make it look more stock. You can solder on the bottom of the fuse box the wiring for the fog lights, everything should be present except for the wiring harness to the fog lights. After you've connected the fog lights to the fuse box you have several options for how to control them. At the BCM you could connect the fog light relay control with the low beam relay control, this would essentially work the same as method one, except you connect the low beams and fog lights at the BCM. The other option is that you buy the stock headlight switch with fog light control. This will not work straight out of the box, to have it work completely OEM the dealership has to reprogram the BCM so that it will interpret the fog light signal and switch the fog light relay control accordingly. However, one could just cut the fog light relay control @ the BCM and T-tap it to the fog light control signal. This way the BCM does not have to be reprogrammed, plus the fog lights will then not turn off when you turn your high beams on. If you do want your fog lights to turn off when your high beams turn on, just add a relay and connect the relay's coil to the high beam relay control

The third option is very similar to the first option, except instead of using a light signal, we pull 1 or 2 wires through the firewall and put a switch behind it, this will look the least OEM of them all. You can however, get the OEM light switch with fog light control and wire the fog light signal to a relay to control them behind the dash and pull the wires for this through the firewall. This would work the same as the second option, this way you don't have to mess with the wiring at the BCM. However, this way you do have to pull 1 or 2 wires through the firewall.

We wired the fog lights to the low beams as described in the first option. Here is a quick overview / diagram of how we need to wire it up:

This is a quick overview of how we need to wire the headlights. You have to connect the right fog lights (left in the picture) to the left side of the car (right in the picture). We did this by guiding the wire up to the right headlight (left in the picture) and then guiding wire along the existing light wiring harness it through the tube right behind/below the grill to the left side of the car (right in the picture). The left fog light is pulled up the headlight wiring on the left (right in the picture). Both of these fog lights are then spliced together and connected at a relay. This relay is switched via the low beam signal which in turn can connect the battery (+) to the fog lights when the low beams are on. The ground is always connected. For the low beam signal, check the wires on the connector on the headlights and test with a multimeter which is the low beam signal wire

Above, you can see the right fog light wiring going up to the headlight wiring where it is guided through a tube that was already present to the other side.

At the left headlight connector we connected a green wire to the low beam signal, we then connected this to the connector allowing us to disable the fog lights if needed simply by disconnecting this connector. (You can see this connector right above the left headlight connector). This signal then leads to the coil on the relay and the other side is connected to ground (leads off to the right along with the batt(+) between the fender and the battery to the fuse box under the hood. The left and right fog lights are spliced together and are connected directly to ground and the other side is connected to the switched battery(+) on the relay.

This is between the battery and the fusebox under the hood. You can see where the wire connects to the battery (+) under the fusebox cover, and the ground connects to the bolt against the fender.