I had this problem with my wife's 95 SL2 a few years ago. The mechanics
told me it would cost something like $200 to fix, so I decided to have a
go myself.

It was leaking right where the tube goes into the firewall, right in the
center of the car. It is difficult to see and reach. I was able to fix it
by applying some black rtv silicone sealant to the area (after letting
everything dry out). I had to rig up a syringe with a tube on the end just
to be able to get it in there. I did it from the passenger side while
lying in a very uncomfortable position.