The heat sink won't go bad unless it's heavily corroded or physically broken. The transistor pack that it cools can die, usually shorted so the fan just runs in high speed all the time. If you're replacing the transistor pack, check your blower motor while you're in there. If the brushes are worn down, commutator grooved, or bearings tight, it can cause extra stress on that module and kill it early.

__________________
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]