The cable could have come loose, the cable could be bad, or the heater element is bad.

The cable goes to the RH side of the engine (facing front), under the turbo. You have to be under the car and look above the starter. Pull the cable off and reattach it (wear gloves or have a good hand/arm cleaner available). If it still doesn't work, pull it all the way off the car and check for continuity on both wires from one end to the other. If you have continuity, the cable is OK, the element is bad. No continuity, the cable is bad, the element may or may not be OK. If the cable is bad, I can sell you a new one that I bought and installed, but took out again because the element is bad on my car. $10 plus shipping. If the element is bad, it's replaceable, but may be very hard to get out. In my case, I just installed a lower radiator hose heater for $25.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer