Alex,
That Duraterm kit will work for W115 and "early" W123. By early, I mean if you have loop-type glow plugs. You can tell because the wiring will be in series with resistor links. I bet Aaron is right and you have the pencil-type with parallel wiring.
If it does apply to your car, see my post and follow-up with clarkb7306:
http://www.peachparts.com/shopforum/showthread.php?threadid=49683