My '85 (617 engine) had the heater installed as a free option when it was sold new (the car was sold/delivered in L.A. by the way, I have the records from new). The actual heating element needed to be installed by the dealer. My '87 (603 engine) had the heating element factory installed in the block and the dealer would supply and attach a cord for free if you asked for it within a year of purchase. Unless it was installed by a PO I doubt you have the heating element installed. As others have said they are not hard to install except that the plug in the block can be very hard to remove and sometimes the car needs to be well up on a lift to be able to get enough leverage. Shop around if you're going to have someone do it for you, you might find a not too busy mechanic who will do it pretty cheaply.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold