1. Remove the 6-pin plug from the heating control relay, turn the ignition key to position "2" and connect the control wire to connectors 1 and 3 on the plug.
2. If the heating indicator light now lights up, the heating control relay is faulty, replace the relay. Otherwise, remove the bulb, check it and replace it if necessary.
3. If the lamp is in good condition, then use an ohmmeter to check the current flow through the black wire between connector 3 and the incandescent lamp; if necessary, check the contact of the lamp with the housing, and eliminate the wire break.
Caution! If the preheating lamp lights for a long time, this indicates a malfunction of the preheating control relay (contacts stuck together).
(Article was borrowed from the website www.mercedesman.ru)