2002 wagon, tail lamp (brake light) failure
 

Brake light in LR taillamp housing is always in-op. It is not the bulb. The shop says the fixture is bad, common problem, and should be replaced. Has anyone found that there is a circuit board or something that can be resoldered or a circuit board that can be replaced, rather than buying a new taillight assembly? 120k miles on 2002 E320S4. Any suggestons as to where to get parts?

Try this. Insert the socket and turn all the way to seat it then turn it back about 5 degrees. This creates a new set of contact points. I tried this suggestion and haven't had an issue with brake light problems since (+2 years).

As always YMMV :D

The problem is not a circuit board but the housing itself. The heat from the bulbs soften the pastic and over time the spring tension for the bulb holder pulls and deforms the palstic until no contact is made. You can dick with it and it may work for a bit but it will need a new assembly.