At 04:57 PM 3/12/06, Jim Beaton wrote:
"1957 MGA 1500 Coupe. I would like to install a third brake light for safety. Do you have any information as to how this light should be wired."

The 1500 model has independent left and right brake lamps which also serve as turn signals. As such there is no form of electrical logic which can trigger a third brake light from the existing wires with perfect accuracy (but you can check the next page for a good try).

The solution is to run one new wire from the brake light hydraulic switch to the back of the car to power the third brake light. See the 1500 brake lights diagram.

Connect the new wire directly to the brake switch to share the terminal with the green/purple wire. Or find the snap connector in the engine compartment near the starter switch with the green/purple wire. Change the single tube connector to a double tube connector and put a bullet on the new wire to connect it there. Run the new wire along the harness under the right side of the chassis past the fuel pump. How you get from there to the new lamp depends on where you mount it and how creative you are.

You can ground the lamp to the body of the car. If you mount the new lamp on the boot lid, or if you don't trust the sheet metal for a good ground connection, you can run a return ground wire to the harness connector bundle just aft of the RR wheel. Locate the multiple black wire connector, change the 4-way connector to a 6-way connector, and plug the new ground wire in there. Alternatively you could ground it on the frame near the fuel tank on the right side, the same grounding screw as used for the main harness grounding point.

Hooking up a third brake light is easier for the MGA 1600 model. This one has the brake lights separate from the turn signals, so the separate brake light wire is already in the harness. See the 1600 brake lights diagram. In this case you can run one or two wires from the new brake lamp to the connectors aft of the RR wheel. Connect the new lamp supply wire to the Green/Purple wire connector. Connect the new grounding wire to the black wire connector or to the chassis of the car.

Here is an off site article detailing the physical installation of a third brake lamp on the MGA.