If the fan runs on high, but not on low, you have a bad relay or a bad fan resistor. The resistor is a real PITA to get to, it's mounted down below the brake booster. If the old fan was seized up, it very likely burnt out that resistor, sorry to be the bearer of bad news!

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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]