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This morning I was informed by another motorist that I have no rear brake lights. The one on the rear window is functioning, but the lower ones don't come on at all. All the bulbs look OK and the other lights seem to work. The swtich of the brake pedal arm is connected.
While I try to make sense of the wiring diagrams, I was wondering if anyone has a suggestion where the fault may lie? I suspect there may be a bad ground somewhere.

try checking the circit boards where the lamp fitting is connected.

mine have developed problems after 20 years and needed a bit of solder.

remember a multimeter is your friend when testing electrical gear.
also if you dont have anyone around to hold their foot on the brake pedal, just cut a piece of wood to lodge between the seat and pedal.

good luck.

I checked the circuit boards on rear lights and two or three of the tabs on the input side for the left rear look pretty bad. I used a circuit writer pen which dispenses conductive ink to repair the tab. When I put everything back together, one light worked for a few seconds on application of the brake. But I may repeat the repair with solder.
I took a multimeter to the board and found some interesting paths, so now I'm wondering if the poblem may be in the board. I also noticed that the horn is not working. My manual indicates that the relay for the horn and stop lights is the same. Unfortunately IAP doesn't have that relay listed in stock. Does anyone know of a substitute relay?

Wouldn't any 30 amp relay work? If you decide you want "new" circuit boards, let me know as I will be selling mine shortly. I'll even throw in a relay

One of the local auto parts store had a relay with same plug pattern, but it was identified as being for the flashers. Another one couldn't cross reference the unit I remove (its factory original). So I guess the question is, do I just match the plug pattern when selecting the replacement?
Also is there a way to check the relay using a multimeter?

Yes there is, it's fairly easy. You will need a multimeter, wires with alligator clips on either end and a car battery. If you look on the diagram you will see a printed circuit off of a relay. This circuit should also be on your relay. The numbers may be different, but that is okay as long as you can match them up with the numbers in the car where the relay plugs into. It doesn't matter if you hook the relay up wrong, it wont hurt the relay, just try again. Okay, I don't remember the numbers exactly but we will use these 30, 85, 86, 87 The diagram is showing you how the relay works, the middle circuit is an electromagnetic switch. (30,85) when hooked up to 12v DC it switches on the second circuit(86,87) So hook the positive of the battery to 30 and the negative to 85(they should be perpendicular to each other, the positive battery hookup is the one facing the other way, I'm not sure which side is negative either 85, or 87 if it doesn't work one way, try the other) Then turn the multimeter to the Ohm or resistance setting and place the probes on the remaining prongs(86,87) or (86,85) the reading should be 0 or very close, if the reading is 1 both times, then the relay is bad. you should actually hear it click once you give it power, its pretty loud for such a small box. Hope this helps, let me know.

Rather than mess around with a multimeter trying to determine which lead goes where, I removed the cover from the relay. Here is what I found:
85 and 86 are at opposite ends of the coil.
30 goes to the body which holds the coil. I assume the coil is isolated from the body. One end of the arm has a spring loaded movable "flap" which is not quite in contact with the end of the coil. A small contact arm is mounted to the end flap.
87 (There are two 87s) are connected to a post which terminates near the end of the contact arm.
When the coil is energized, via 85 and 86 and it draws down the arm so there is continuity between 30 and 87.
If anyone is interested, I'll try and post a photo of the relay.
I'm wondering if there is someway to check the corresponding sockets on the relay mount to determine of the problem is elsewhere? Shouldn't I be able to measure a voltage between 85 and 86?

just jumper "30" and "87" (per your description)with a short 10 amp fused wire...lights should come on. Then you know the lights work

I had badly corroded light sockets and connectors...replace all bulbs while out with some heavy duty filament types

Also, check brake light switch (touches pedal arm)

As it turned out the relay has nothing to do with the brake lights. I had miss read the poor copy of the manual I had printed out from the Car Disc CD. The relay was for the brake fluid level.
I traced the wiring to fuse no 4. The wire from the brake switch to back of the fuse was OK. When I push in on the connector the to the fuse lead, the left rear brake light came on, but not the rightside. I cleaned the connections on the rightside cluster (and the leads on the four wires for the ground point near the rear hatch latch). When I checked the brakes again, neither side worked.
Then I played around with the connections at the fuse box with no success. I did notice that they all dark brown. But they are too small to easily clean.

Get some electronic connection cleaner, if that doesn't work try carefully using a knife to scrape, but not too hard. Also you can use copper connection grease and a Q-tip or toothpick. Or you can just buy mine.

you checked the trunk ground too?

Next I'm going to check the connector between the fuse box and light cluster. Inspite of their apperance, I think the fuse box leads are making good contact. This is because two wires exit the fuse lead and I think one of them is for the brake light on the rear window which does work.
I did the clean grounds on the trunk and the lights seem to improve. On one of the circuit boards for the light clusters, the connections were a dull silver and a couple were partially burned off most likely as a result of shorting. I cleaned them with "Mother's Mag and Aluminum Polish" (to my suprise, I discovered they were actually coppered colored). For the burned off leads, knowing my soldering skills are lousy, I used adhesive back copper tape which was almost exactly the right width for the application. This tape is used in a type of stain glass know as "Foiling" and is perfrect for this application.