My 83 300sd came with a factory installed heater, located on the RH side of the engine (RH side as you face forwards). The cord is hidden in a flap under the bumper.

If yours is busted or removed you can get replacement kits with instructions for either a block (better, more difficult to install) or hose mounted (inferior, easy to install) block heaters at www.**************.com

If you go for the local parts store option make sure it is properly sized for the metric hole into which you will put the doubtless inches measured unit....

My 83 300sd starts fine down to about -12 Centigrade before a block heater is necessary.