The REST function is Residual Heat Utilization. ( MB likes abbreviations )

It works like this: Drive the car, park and shut off the engine. Press REST and the heater valve opens, water pump turns on and blower runs at low speed. This keeps the car warm until the engine gets cold or you come back.

If you do replace the pump, be sure to add a inline fuse. Older systems didn't have a fuse and if he pump locks up, it will literally burn down the heater control unit.

Also do a search for " blower motor resistor fire ", some cars would burn due to leaves getting on the blower resistor.