I got the 124 dash light upgrade kit sold by these guys:
http://www.**************.com/
IMHO it yielded a significant improvement. I also bypassed the dimmer switch because it wasn't working right, but that didn't seem to make the lighting brighter, it just makes it what it was on the brightest setting of the dimmer. Some people say these upgrade bulbs don't work, but they certainly did for me. I have also heard that the earlier 124s (like my '87 300E) have dash illumination that is worse than the latter ones, and these not improve lighting on later 124s nearly as much as on earlier ones, but that is just a guess.