Another interesting mod would be to add a 120V normally-closed relay, so plugging in the block heater automatically interrupts the starter circuit and prevents driving off with the cord connected. Some electric cars have interlocks like this.

Of the four diesels I've owned, my current 240D is the only one that came with a block-heater. Though in the four years I've owned that car, I've never driven it in cold enough weather to need it.
Many years ago, I installed one in a friend's '80 300D. That job was an ordeal in itself!

Happy Motoring, Mark

FYI
 

The cord should be inspected every year, and repaired or replaced at the first sign of damage.

More examples

Block Heater Fire - Google Search


.

Beware of the "www.Croberts" link on that page, the one right below the pics. When I clicked on it my ESET intercepted and reported a virus.

... perhaps new heater cords are in order?
Mercedes Diesel Block Heater Cord OM60X Engines New | eBay
I'm just sayn...