Volvo Repair: volvo brake light problems, volvo 940 wagon, bulb failure

Question
Hello and thank you for your time.

My 1993 Volvo 940 wagon's brake lights stopped working.
I checked the bulbs-everything good
I checked the fuse located behind the ashtray-good
I replaced the stop light switch
As best as I know how to, with a volt meter I:
1. Checked if volts were going to the fuse, gives a reading of 12 (if I remember right)
2. Checked if power was going to the stop switch-good
3. Traced wires to two or three different plastic plug-in's
that pull apart-seems to be okay.
4. Checked volts to the back taillight terminals themselves- NO POWER THERE
5. On the fuse box behind the ashtray I found a series of ten different colored boxes and cylinders of various sizes. One of them is the bell to let you know your keys are in the ignition for instance. I found out that one of these boxes (it is actually a red cylinder) has something to do with the brake lights.

I don't know what else to do. I don't even know if I did all of the above things right.

Can you help me?

thanks
Robert

Answer
The big red round relay is a bulb failure sensor, that is probably the source of your problem. New they are about 90 bucks. You can pop off the cover and look for bad solder connections(turn it upside down and look in the middle,that is the most notorious place)Locate the bad solder connection and re solder, it should be fine.