I used to have one fitted to a 109 SIII. Easy thing to fit, just bolt it to the inner wing, pipe it into a heater hose and bring the socket out to a sensible point. Great thing in the winter - heat the engine for half an hour before seting off. Saved the running on choke and heat straight away meaning no misted windows. If your very clever you can add a timer that fires up the cars heater fan to warm the interior before you get in. Just remember to unplug it before you set off!
Having said all of that I would look for a S/H one as they are a bit pricey when new!!
ZD