With regards the likely leak in the fuse box area, this is well documented on the forum, and I would say highly likely you have rust - through in this area. The rust usually permeates into the firewall, which then leaks water to run down into the drive side footwell, so you will likely also find a rust nest under the foot rest and surrounding area. The problem is made worse by the very thick sound deadening used on these cars, which acts like a sponge and traps all leaks nicely, this is what leads to rust in the footwell.
There are no shortcuts to fixing this, and I doubt most shops would want to touch this job anyway as it is a PITA of note. The access is super tight, plus you have the hood release cable grommet in the way, not fun. If you want to do a good job here you need to lift up the fuse box and remedy all the rust, as well as fashion a drain tube (which Benz failed to do, sadly). This will then carry future water ingress into the wheel well as opposed to into the cab, of course assuming too that you also remedy the fireall leak and related rust you likely will find in the floorpan(s).
Cheers
