I have an odd #1 plug wire on my 1975 450SL, that I need some help with. It has a sliding rubber thing on it with a wire coming out of it that goes to some sort of electrical socket by the power steering pump. It appears to be some sort of test site plug. Is it where a MB mechanic can plug in some sort of tester to check firing order and/or other componets on the car? As I need to replace #1 plug wire is that extra thing a necessary item (is it vital to car operation)?
Thanks for any help.