W124 blower motor regulator

[MacMade300]

What actually goes bad? The heat sink or the wired sensor that mounts onto it?

Unless it’s cracked can the sink even go bad?

[Diseasel300]

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.