I had the same problem with my 240D, It all but quit lighting up one day so I pulled the dash and the rheostat was all melted to hell. I knew I wasn't going to find a new one of them (cheap) so I got a piece of wire and shorted the contacts where the rheostat used to be, now I get full brightness all the time. And trust me full brightness isn't much .

But first narrow this down, Are all the instrument lights dim? Even the A/C controls/hazard button backlight/shifter light. They should all be controlled by that one rheostat.