My 70 cutlass's tail lights are all on but the far driver side is always super bright. Bright like its always set to "break pressed position". The other lights have a transistion from normal run to bright run when break pressed. I changed the 1157 bulb but that didnt change anything so looking for some ideas as to next steps.

Any ideas?

 

Possible ground problem.

 

Possible that its wired wrong, or the bulb socket is allowing the lamp to be put in backwards?

 

^^^This.

With the tail lights on, have someone press the brake pedal. The side with the bright taillight will turn off when the brakes are on. This is the sign of a bad ground.

When the ground is bad, the tail light circuit grounds through the brake/turn filament in the 1157 bulb. This causes both filaments to come on, which is why it's bright. Press the brakes and now that alternate ground path has +12v instead, and with +12v on both sides of the bulb, you get no light.

 

I tried the theory of pressing the brake to see if the other three lights went STOP bright and the far left goes dim and that failed. When depressing the brake all 4 become the same STOP bright.

Any suggestions? I'll try to pull out a wire diagram and see where the ground for the tail lights are.

 

check all four light bulbs to see that they are the same number. If they are not you
have feedback from the odd bulb. basic test question for electric diagnosis.

 

Update
All 4 light bulbs are the same number. One thing I did notice is that the wire connectors for the bulbs are 2 wire connectors simular to what a 68/69 require. The tail light wire connecter that I see (@ opgi) have three wires "brown/black/yellow". Before shelling out 80 bucks to replace all 4 would that cause the problem?

I also see some really shabby splicing work with the wire connectors going into the harness clip pig tails for the tail lights. I will try to clean that up first as well UNLESS someone says that these 2 wires connectors are incorrect.

 

If you have the correct bulb (1157) then the problem is wiring. The "tail light" or "parking light" uses the low intensity filament in the bulb and the "turn signal" and "brake" use the high intensity filament. If under normal conditions one bulb is too bright, something is energizing the high intensity filament.

If your wiring is chopped up, you need to restore it to factory. If there are stripped wires inside the "really shabby splicing work", they could be shorting together and that can feed power to both filaments. If the bulb is not fitting into the socket properly the bulb contacts can short the wiring circuits together and turn on the bright bulb. You could also have a bad bulb out of the box with a broken filament touching together inside the glass.

It is a pretty simple circuit but you need to make sure everything is working properly first. Then troubleshoot.

 

EIGHTY BUCKS!?!?!?!?

Geeze, try NAPA or an online source like this.

The third wire is simply an additional ground wire. Your stock sockets are swaged into the taillight housing and not easily replaceable. The ground path is through the housing, into the bumper, and into the frame. Rust and corrosion can cause a ground problem. A dedicated ground wire to the frame bypasses this nastyness - assuming you can get the original swaged sockets out and new ones in.