How to Fix a Bad Ground Wire for Brake Lightsby K.K. Lowell
A bad brake light ground connection will result in the brake lights not working, or power crossfeeding through the tail-light circuit, preventing both tail and brake lights froms working at the same time. Repairing the connection is a project that is well within the range of the do-it-yourselfer.
Locate the ground connection for the brake and tail lights. This will require you to remove the trim panels in some car trunks, which you can in some cases accomplish with a Phillips-head screwdriver or a small socket. The method will depend on the make of the car.
Remove the screw holding the ground connector to the car body.
Using sandpaper, remove any rust from the connector and the contact area on the car body. If the connector is too rusty, replace it with a new solderless ring connector.
Reattach the ground connector to the body. Use a new screw if the old one is very rusty.