Parking Heater

I fitted a Webasto to a 92 diesel a few years ago. It's like any aftermarket item in that it can be done, but to get it to look original is more difficult. There is a standard place for the controller in the lower console behind the gearstick. And you need to get hold of the one which fits or it looks untidy. There's also a little microswitch which needs to be fitted to the heater control valve behind the dash.

Other than that the standard G has a location for the webasto and you can get a bracket from MB to fit. The easiest way is to contact a Webasto dealer and ask them to fit one but tell them you want it to look original or Buy or the parts from MB

Buying s/h is risky as you might not get all the parts you need

If fitting a second hand one, make sure you see it working before it is stripped off the vehicle. I would not trust a dismantled one unless it is has warranty and worth the paper the warranty is written on. 

Any mechanic should be able to install one provided you have full fitting instructions. If these heaters are not serviced properly they can be a pain to repair so my suggestion is to buy a new kit.