I managed to repair an old rheostat I had for my 300D, simply by (CAREFULLY!) disassembling it, and using electronics cleaner to make sure the contacts are shiny and clean. Granted, if the guts of it are falling apart, this trick won't work - but it did for me. A little brillo to gently scrub any corrosion off the contacts can help, too.

I did this to mine, let it dry off, carefully reassembled it, and now I have working rheostat again. If all else fails? The soldered wire trick always works.

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]