This 1995 Chevrolet S10 pickup came in with the complaint that the park lights would work until the brakes were applied or the turn signals were switched on. In reality none of the rear lights were working as they should. It all sounded like a bad ground to me so I wanted to start by checking the ground at one of the bulbs. There are two phillips headed screws that hold the light assembly in place. Once removed the assembly will pull out.
The socket twists to unlock and then pulls out of the lamp assembly. Note the bulb elements in the following picture. Although only the park/tail lights are turned on both elements are glowing and neither one is bright. This is due to a poor ground condition.
To further prove the faulty ground condition I back probed the black ground wire where it enters the socket assembly. Note that my test light is illuminated in the next picture. This is due to un grounded power back feeding the ground wire.
This was a pretty easy diagnosis when I spotted the cut ground wire under the driver’s side rear of the truck.
The factory ground bolt was heavily rusted and would not come loose without a lot of effort. It is just barely visible at the center of the next picture.
Instead I drilled a new hole in the frame.
I lengthened the damaged wire and added a ring terminal. I secured it to the frame rail with a self tapping screw. It does not look like it but I did wire brush down to clean metal. Afterwards I sprayed some black paint on the connection point.
Now the stoplight element is bright when the brakes are applied.
This customer is going to have a lot more problems with this truck due to heavy rust on the underbody.