Don't even waste your time trying to fix one of those. Either bypass it or replace it. The heartache of failure simply isn't worth it. Not sure how hot you want your car, but my SDL doesn't have a working AUX pump and I don't see any reason why it should. Gets plenty hot even on the coldest days.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]