Mercedes designed the lock that way so that you have to lock the door with your key from the outside, thus, it is (almost) impossible to lock your keys in the car. Its a great idea.
The wood might need to be glued back down or the tabs that hold it in place may be out of whack or broken.
As for the heat issue, its not a gas engine, it takes a diesel a little while to heat up enough to feel anything. (Because they are far more Efficient than a gas engine, less waste heat) My car takes up to 5 mins of driving in the winter before I get much usable heat.
1986 is a good year for the diesels. The om603 6 cylinder diesel is very very reliable if properly maintained and NOT OVERHEATED. Overheating can crack the aluminum head on the engine = big expenses. Maintain the cooling system properly and you'll be fine/it will run for a looong time.
The "bad diesels" are the 1991-93 era 350SDL 3.5 liter engines. They have a tendency to bend their rods and fail.
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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)