There exists a cheaper aftermarket reference sensor which typically sells for $79.95 a piece (you still need two). The difference between them is the casing, it’s plastic instead of aluminum. But, keep reading… there’s an even BETTER option. There is a BOSCH replacement sensor that was used on the ~1983-1985 BMW 323i that are IDENTICAL to the pricey $140 dollar ones, even have the aluminum casing, but… they’re also half the price. Expect to see these priced anywhere from $59.95 to $89.95. As these cars begin to get more rare, I would expect these prices to eventually go up, so if you think you MIGHT need to replace them in the near future… I would get them. The BOSCH part number is “12 14 1 708 618” This part can be ordered anywhere German / European Bosch parts are sold (or your nearest BMW dealership).

It’s worth mentioning that the ONLY difference between the ones sold for the Porsche 944 and the BMW 323 is the length of the wire. Quite literally, the absolute ONLY difference is that the wire is about 1 and a half inches longer on the BMW sensor. That’s it!!! The sensors, plugs, and mounting are otherwise 100% identical. Now you can put that $100 you saved in a new sunroof seal!!!”

excerpt from http://www.pontiacperformance.com