There are two pumps one is for the closing assist (pulls doors and trunk lid in) those pumps fail regularly. This pump is located over the rear wheel next to the gas tank, remove both liners to get easy access(wheel well & gas tank/package tray)
The other pump controls , the trunk handle, lumbar, headrests if you dont have electric rear seats, central locking, and provides vacum for the HVAC. It is located under the rear (passenger) seat. This one does not fail as often. There was a bulletin about 95 cars having a bad hose inside the closing pump and a hose was available from MB, but that would indicate all doors would not pull in.
I am a little leery about buying one one bay, you could get one as god as yours. I purchased a new one at MB and it failed 3 years later, they did replace it, since they installed it.
George Barrett rebuilds them and gives a warranty, he advertises in the STAR