Rear Driver's Side Taillight

Hey guys. I have an unusual problem with my 1990 Geo Prizm. When the taillights are off and you step on the brake, the left brake light turns on just fine. But when the taillights are on and you step on the brake, the left brake light will not come on, the light just goes out. The right side works just fine in both applications.

This is a classic case of a bad ground to that light. Either corrosion in the socket is not allowing the bulb base to contact the socket sides or the separate wire that carries the tail light fixture ground back to the metal of the body is open. The attachment lug could be corroded or the wire may be broken.

A few minutes with a DVM will determine what the problem is. Hope this helps.

Could you explain what a DVM is, please?


Digital Volt Meter.

Change the bulbs out (you didn’t say you did this).

Suppose the ground point of a dual element bulb is broken off to ground? When the parking lights are on, the current will travel out the brake element and ground through the other bulb. Now put on the brakes and both the brake filament AND the parking filament are seeing 12V …balance voltage …the parking element will go out with no where to find ground. With the parking lights off, either will find ground through the other filament …which is ganged with either the other brake elements …or the parking elements.

It can drive you nuts if you don’t understand the current flow and the common points.

Either the fixture ground (the housing) is bad …or the internal common point (both brake and parking element in the same bulb meeting at a common point) is broken.

I forgot to mention that I have changed the light bulb.

Is this a complicated procedure?

Then the fixture’s ground is probably corroded.

Try applying brakes with parking lights off in the dark. See if the parking light elements (it may be the turn signal) in the front lights up.