I had this problem on my wife's 95 sl2 a few years ago. the mechanics told
me it would cost something like $200 to fix, so I gave it a try myself.

It was leaking where the tube goes into the firewall in the middle of the
car. It is in a very difficult position to see and reach. I fixed it by
applying some black rtv sealant to the area. I had to use a syringe with a
tube hooked to the end of it. I did it from the passenger side while lying
in a very uncomfortable position.