1980 380SLC, 1989 300CE, 1973 220D (work in progress)
If you're handy with a soldering iron then you can just buy one of these and splice it on to the existing cable: Thermistor 3K NTC TT3-3KC3-BG | eBay
The sensor is just a 3k Thermistor (it should have a resistance of 3 kOhms at 25 deg C).
Make sure you encapsulate it to protect it from the elements. I've done this on a couple of cars, works a treat!