Hate to scare you, we have a 98' C43 sitting on our lot for 2 years now, abandoned now because the BAS module has died and a replacement one is either unavailable or ungodly expensive. Been trying to get in contact with the owner, (on here btw) who is Mia.

.. But on a brighter note, you can try and install a brake lamp switch and see if that fixes it. The switch has 2 switches inside, so not always can you identify a faulty brake lamp switch by looking to see if the lamps actually function. That is the most common failure, that is the one part that you can take a gamble and install with a good chance of positive outcome. Part isn't expensive either.