This is something you can rebuild yourself, but to answer your question there is a place out of WA, merc.....source.com that sells just the motor sans squirrel cage. They're more expensive than just about everyone 'cept the dealer.

I have a spare that I'd sell you, but don't think it'd help cause I'm not sure it works or how much life the brushes have left, etc.

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)