brake lights are out but not...

[Wonderingraven]

Here is a weird situation.
New bulbs has already been replaced and tested.

1986 mk1.5.  ca20e

When I hit my brakes, the trunk brake lights come on but not the rear tail lights (stop indication)
checked fuses and all is good.

Other than the relay in the rear what else can I check?
it seems between the fuse box and the battery but if it was something there wouldn't the trunk brake light not work also?

[JonB]

Your saying the third light comes on, but not the two brake lights in the tail light assembly?

[rednucleus]

Seem to recall this can have something to do with the sensors that detect burned out bulbs?

[Wonderingraven]

Your saying the third light comes on, but not the two brake lights in the tail light assembly?

Yeah that is correct.

Seem to recall this can have something to do with the sensors that detect burned out bulbs?

Where is that located? 
this might be a good place to start looking at.

[JonB]

Are the correct bulbs installed? Some tail/brakes have a three terminal bulb where others just have two - iirc.

Other than that, its multi meter time i guess.

Not sure the s12 has, light sensors per say - just simple inductive things, and i think they only do the headlights.

[rednucleus]

They do tail lights as well - your dash should have warning lights for fronts & backs
Post Merge
https://club-s12.org/forum/index.php/topic,35040.msg368752.html#msg368752
searched light sensors

[Wonderingraven]

questions is if it was the sensor wouldn't all the brake lights not work?

[JonB]

They are left and right sensors.

After 3 s12s, and 19years of ownership, i can only recall them ever telling me about anything but headlights. Which is not to say they dont do the tails

[deucekazoo]

I would suspect a ground issue. I was looking at the schematic and it does show a  TAIL AND STOP LAMP SENSOR on the schematic but this sensor also controls the 3rd brake light. The schematic shows the 3rd brake light having a different body ground than the rest of the tail lights. See if you can trace a black wire from the lights and where it is grounded to the body. The schematic shows two terminals for ground and it calls them 131M and 162M. On page 775 or EL-202 of the service manual it show the ground layout. The 162M ground is at the back by the tail lights, in the center. The 131M ground is shown by the RH door rail. Hope this helps.
Post Merge
Also depending on if you have a coupe or hatchback the sensor module is 163M. Its located in the center by the taillights in the coupe and rights side rear in the hatch. It should be a 8 pin connector.

[rednucleus]

I'll bet you a cold one it has something to do with the sensor, especially if it has an 8 pin connector!

[Wonderingraven]

This gives me more ideas to try thanks ^^ will give out more in the outcome.

Just feels a bit confused but yeah it's true the 3rd light is on a different ground from the tails.
so my original hunch was a ground disconnected somewhere or broken.

[Wonderingraven]

Ok!
For a bit of an update.

The little box next to the fusable links by the battery "headlight switch"  is the label of the box.
THe connector was covered in some engine grime (not even sure how oil got there) or if it was something else not fully sure but tested it with a multi meter and turns out it shorted it self out.
I have an extra one from a few cars I've pulled in the past for cases like this and swapped it out and boom it works again.

Thanks again for the assurance that this is a major common problem and now noted for the future.

[rednucleus]

So the one marked headlights fixed the tail lights??

[Wonderingraven]

yeah, oddly enough. that is all I've replaced.