MGB GT – Adding Third Brake Light

Step 1 – Obtain 3rd Brake Light #

The starting point is getting a suitable LED third brake light. Lee Fox has identified that getting these from a Volvo works. A V70 (mid 90’s through mid-2000’s) works. Also an S80 from 07 to 11 or similar model is a good starting point. John Mangles kindly got one for me from a recent junk yard trip.The part number is 8693988 and seems to be actually made by Hella.

Now, this is going to have way more plastic than you actually need. For an MGB GT some trimming is going to be needed. For this I found that a hack saw was sufficient. Also just a hacksaw blade on its own for some hand sawing works. This plastic is not hard to cut. If you have a smallish multi-tool like a Dremel or something with a cutting wheel, that might also be a useful way to go. However, I don’t have one of those and the hacksaw blade work just fine and gave me plenty of control.

Step 2 Removing Mounting Brackets #

My first move was to remove all the mounting pieces. These that are shown on the right in the original picture. There are some clear tabs that stick through the back and hold the clear plastic lens in place. So, I left about 1/10^th of an inch gap between my cut and the back edge of the main light part. You can see the mounting parts removed. Don’t remove the electrical connection part! If you look closely, you may be able to see the bits I left in place.

This was relatively simple to do and I used the hacksaw for both of the end pieces. It was a little more difficult to get the center piece, so I switched to using the hacksaw blade by itself and wearing a work glove this was pretty easy to do and to control the blade while working close to the electrical connection.

Step 3 Remove Excess Plastic #

Next up was to remove the excess plastic.On the donor car this would shield the light from its mounting point to the actual window. I decided not to get too close to the front of the clear lens. I used an obvious edge point that would give me about ¼ inch of material in from of the lens. Marking the ends with a pencil I then drew a (relatively) straight line across this piece with the pencil.Then I followed the line and cut the whole thing using the held in hand hacksaw blade approach. This worked pretty well and again was easier than I expected it to be. Took my time and followed the line and things seemed to go pretty well.

Step 4 Testing & Establish Negative/Positive Wires #

Having done all my cutting I did a little sanding using a small block and some 150-grit sand paper. I didn’t need to do that much sanding but I just wanted to make sure that I didn’t have any super rough edges that might catch or cut me. I also then thought that I should test the light. Probably should have done this before starting, but better late than never, I guess. As I have my Durango on a battery maintainer, the battery posts are easy to reach and I could use them to test the light and figure out which wire was the ground and or positive. The light worked and the black with white stripe turned out to be the ground. I was actually quite surprise how bright this light is.

Step 5 – Wiring the Third Brake Light In #

That was the quick and easy bit. The fitting and wiring took me longer as I took my time and wanted to make the wiring as neat as possible. I also test fitted the light multiple times to see if the orientation made any difference to out the back for visibility. With assiatnce. I used a UPS 12v battery I had lying around. Having tried a few things, I decided on mount with the curved pieces sloping down and putting the electrical connection to the passenger side of the middle.

I routed my wiring across the back edge of the headliner and then down the inside of the c-pillar trim. I was able to remove this piece of trim and then route the wires along the outside of the wheel well and out down to the passenger side light cluster. My GT has a lot of custom electrics, having been owned by Rick Astley, of MGB Electrical book fame, before me. I did some tests to check the wiring as there are relays there for both the brakes/rear lights and the turn signals.

You can basically connect the ground wire from the third brake light to any suitable ground. This might be existing ground wires or could even be to the GT Hatch mount bolts for example. I chose to run the wire all the way down to the rear light cluster to get ground and the necessary connection into the brake circuit.

The positive wire needs to connect into the brake lights circuits. Regardless of year, this should be a Green wire with a Pink tracer that connects to the brake light bulb. The bulb gets ground from the light unit itself.

I used red as my positive wire extension and white as my negative wire. So the white wire went to a suitable ground and red positive wire joined into the brake light circuit e.g. should be Green with Pink tracer wire.