[Avocet]

I think it's the largest of 4 black plastic boxes under the dash, passenger side, immediately behind the glovebox.

 

[Avocet]

I'd caution against bringing the car up to temp every few days. My 164 had been laid up for a number of years and the previous owner had "run it up ever week or so". When I got the car, there had been so much condensation in the top of the engine that the backs of the cam lobes were rusty! The oil stank of petrol too, and was really watery. I don't think he'd run it until the thermostat opened and the fans cut in, so mine was an extreme example, but without the engine doing any decent amount of work, the oil may never get hot enough to evaporate off the excess petrol that gets past the piston rings each cold start.

 

[Avocet]

Next time you dip the oil, just have a sniff of the dipstick. You'll soon know if there's a problem!