I second the recommendation for a good inspection of the power cable. It plugs into the heater directly above the front edge of the starter, immediately underneath the turbo heatshield; visible/accessible from underneath the car.

My '83 300CD has the factory heater and is a mostly Texas car, imported from Maryland in '94. I thought "cool, a block heater" when I bought it last month and then I looked at the crumbling, dry-rotted insulation on the cable near the block. The cord near the front of the car was flexible and okay.

No, I didn't have much intent to use a heater on a garaged car in south Texas, but the cord would have created some fireworks had I plugged it in.