I've recently been having trouble with the brake lights. I've had intermittent trouble with the passenger side one in the past, but seem to have that resolved. I've added an additional ground since I know that the early lights have had some trouble there. Also, my rear side markers are not illuminating, but I believe that is a different issue. Here is what is currently happening:

I noticed that the "check rear lights" was coming on with the brake lights. I assumed it was the passenger side only, given its previous troubles, but then discovered that both rear lights were out. The high mount center light works just fine. Now the check rear lights only occasionally comes on...but the lights are always out. I haven't dived into it yet with the multi-meter, but I did pull up the wiring diagram from JTIS. It appears that one side is fed directly from the body processor module and one side from the lamp control module. Is the correct, a typo, or am I just reading it wrong? The diagram does seem to indicate that the lamp out indication comes from the lamp control module, so does it seem likely that this is the culprit for everything? I see used ones on ebay for around $30, so that would be an easy fix. Also, I've checked fuses and relays, and they are all good.

I've also read of issues with the brake pedal switch, but my cruise works fine and the high mount light works, so I would imagine this isn't an issue. Correct?

Any suggestions to start troubleshooting this?

I've attached the wiring diagram for reference.

 

I'm certainly no expert here, but the drawing makes no sense, I think the lower lamp unit detail should read from the top:-

DI
STOP
TAIL
REVERSE
FOG

Not

STOP
TAIL
REVERSE
FOG
DI

 

Yes, there are a few goofs in JTIS - that's one of them. Most of the MY get it correct.

Power for the high-mount stop lamp doesn't travel through the lamp module whereas the rear side markers do, although they're not monitored for failure.

Since you've addressed the lamp cluster grounding issue, I'd pull the lamp module and look for signs of a fry-up - it's easy to get to under the trunk fusebox lid and the two halves of its container are simply clipped together. Check also the yellow connector for corrosion, loose wires and bent pins.

You have checked the bulbs and their fittings? A close relative of mine got caught diagnosing two dead dip beams on his daughter's A-Class It definitely seems to eat them.

Cheap enough module to replace, although it's an easy fix if you're keen.

 

So I pulled the Lamp Control Module to see what was going on with it and see if it could be fixed. When I opened it up, everything looked great and I didn't see anything that looked like it had fried...but it did smell a bit burnt. I bit the bullet and just decided to buy a used module from eBay. That arrived today and I popped out the yellow cable and put it in the new module....and no change. I was a little frustrated and pulled the cable. I messed around a bit and decided to spray the connector with some contact cleaner. I also decided that since the old module smelled a bit, and I now had a different one, I may as well use it. I plugged it back in...and everything worked! Success! Out of curiosity, I plugged the old module back in to see if the connection was the only problem, which would have made me feel a bit silly, but the module seems to be bad also, as it stopped working. The new module and cleaned connection were the solutions. I did discover that the passenger side bulb had burned out (again), but I had a spare on hand, so I'm all back up and running. I'm not sure I will ever know why the passenger side seems to burn out at a much faster rate than the driver's side, but I'm not going to worry about that.

 

Well done on the fix. Agree with you that sometimes we’ll never know what the cause of your bad module was, but it’s working now so no worries.

As for why one side seems to fail more than the other, this X100 seems to have many of these funnies - door switches seem en vogue nowadays, suspension, and not to mention weird tyre wear. But I’m pretty sure I know why the driver’s seat wears out faster than the passenger’s! LOL!

 

Thanks for the update.
The pins on that connector seem a little under-engineered for the load that they carry, IMO, but it doesn't generally flag up as causing problems.

Upside is that having the spare module means you can investigate its failure once you've crossed off all the items on your 'stuff that needs attention on my XK*' list