Headlight/Side Marker Modification

For All 1983-88 Cougars / Thunderbirds

Page Revised: 10 May 2019


Article by
Eric Dess

There's one place on these cars that can always use a custom touch, and that's in the area of lighting, particularly with the headlamps. The 1983-86 models have the standard 4x6 low and high beam sealed units that had been used for many years prior. One nice thing about that standard is how the lighting pattern was consistent across vehicles. While that's great, there's always room for modernization and improvement. The aero Fox Cougars saw a dramatic change in headlight design, beginning with the 1987 refreshening, with the new flush headlamp system and replaceable bulb. While a little more limited in upgrading, there's still something that can be done if you want to make your Cat stand out at night.



H4 Headlamps

The H4 conversion is the next step up from stock headlamps. The H4 headlight is relatively new to North America, although they have been available in Europe since the 1980's. The idea is that they combine the size and shape of traditional 4"x6" sealed beam headlamps, only with a replaceable bulb. Standard halogen bulbs are available, as well as hyper-white and xenon-imitation bulbs, and even full bi-xenon HID bulbs with conversion kit.

Unlike sealed beams, the H4 lamp will continue to function even if the lens is cracked or shattered because there is no high-pressure gas inside. But the real boon to H4's is their light pattern: it works almost like a fog lamp, with excellent side-to-side lighting and the virtual elimination of a bright spot for the low beams. This is truly a blessing for those that want to upgrade to something more modern. Some versions are DOT street legal, while some are intended for off-road use only, so be sure to check first. Legality with H4's is a delicate situation.

H4 Wiring Harness

A few things to note about H4's: they work off a totally different setup as far as wiring configurations. Fortunately, most H4 kits will come with a minimal conversion harness that plugs right into your own stock harness, so be sure that you get this with yours. Better yet, if you can, buy a separate harness like the one shown here. It's the ultimate in wiring security because it lets you run higher wattage bulbs (don't even try it with stock wiring!) and it also pulls its power directly from the battery, ensuring a full 12v at each lamp. As far as the bulbs, the standard 60/55w bulb will probably be too dim. The 100/80w are good, or there's even a 130/90w bulb available.

H4 Wiring Diagram

This illustration shows the wiring changes needed to accomodate the H4's bulb configuration. Since the blades are in different positions vs. the stock setup, you will need to make sure that everything is wired for the H4's. And if you are going to use H4's for your high beams (inner lights), you may need to bend the top (middle) tab down on the bulbs.

H4 Diamond-Style Headlamp

Similar are the H4 crystal headlamps. These have a totally clear lens with a patterned internal reflector. Good crystal H4's come with a bulb cover to keep from blinding drivers' eyes in oncoming traffic. Again, check the legality in your area first. They are also available with black reflectors for a totally different look. These lights are readily available eBay.

H4 Projector-Style Headlamp

Another choice would be projector conversion headlamps. This uses a standard 4x6 headlamp housing and inserts newer projector beam technology inside. We've talked to a few people that have tried using them in a Cougar and they didn't recommend them, as the beam was nearly unusable, but you may have a better experience.

LED Projector Headlamp

Fast becoming another option is the LED projector conversion headlamp. The combination of a projector lamp with LED accents provides a great look for street and show. Multiple colors are available for the LED accents, and some you can even convert to use as turn signals. Be sure that they are DOT street legal if you're going to use them for daily driving.

Full LED Headlamp

We now have the luxury of installing full LED headlamps. Obviously there would be no worries about ever changing bulbs again. And the light output is going to be very, very bright. Plus they'll run a lot cooler than traditional bulb-driven units. They are more expensive than any of the other lamps listed above, but longevity and brightness are good payoffs for the price.


As you know, your headlamps are pretty unique to the car (well, the 1988-94 Lincoln Continental had them too), so there is no swapping of Mustang headlamps into your car, we're sorry to report. But you can swap out bulbs—carefully.

9004 Xenon Bulbs

Your stock 9004 bulbs can be replaced by any number of aftermarket bulbs, including hyper-blue, hyper-white and imitation xenon blue. Your local parts store probably has quite a few types on the shelves as you read this. Generally, the better bulbs will be more expensive. You may end up spending $50 or so to change your bulbs, so be warned. And there is no guarantee that the hotter bulbs won't melt your headlamp housings. Use at your own risk.

Bi-Xenon HID Bulb Conversion Kit

You might be tempted to buy a full xenon/HID conversion kit for your headlamps. In theory, xenon bulbs will never burn out and will only need replaced if they're smashed or damaged. But true xenon/HID setups are amazingly complex and require unique wiring that is much better than what came in your car. The way the system works is that a high-voltage ballast ignites each xenon-filled bulb when the headlights are switched on. The effect is similar to sodium vapor lights used in gymnasiums: initially the lights are dim but rapidly get brighter. Each ballast throws off roughly 20,000 volts, which is enough to do some damage to your body. In other words, this isn't something you just play around with. You can use an HID conversion kit, which comes with everything you need including wiring, bulbs, and the ballasts. They are available on eBay or through JC Whitney.



The stock side markers are, shall we say, rather large and not congruous with some paint combinations. Unfortunately there aren't many choices for customization but you can always paint them.

1983-86 Side Markers

For this custom side marker, it was simply done by painting over a custom Cougar logo and clearcoating it. The trick is to not layer the basecoat on too thick if you want some light to shine through the painted part. Check the opacity of the paint by shining a flashlight inside the marker as you paint. You should find that 2-3 thin coats are about what you'll need for lighter colors, and 1-2 coats for darker colors.


Again, there are no aftermarket clear markers for these cars. Still, they are semi-clear from the factory which is a good thing, because you can make your own clears if you're careful enough.

1987-88 Clear Side Marker Conversion

Start by removing the marker assembly. Using a Dremel or similar cutting tool, hack apart your perfectly good stock marker, then remove the amber reflector from inside. Then seal the whole thing up with copious amounts of silicone, and there you have a custom factory-style side marker. Simple, easy, and best of all, free of charge...if you don't ruin your marker in the process, that is.