In the engine bay along the firewall to the right of the ABS controller (which is the most menacing looking thing on the firewall). You'll see a small square device about 4cm per side with 4 flathead screws. Unplug the connector, remove the screws, lift the round thing in the middle, replace the cartridge, yadda, yadda, yadda. Keep track of the order of the washers. You shouldn't lose more than a couple of teaspoons of coolant.

Sixto
95 S420
87 300SDL