It's been a while since I did wiring on Mercedes so if memory fails
you know the drill.

Somethings gone short or to ground in the #15 fuse circuit.  Get the
wiring diag out and find everything that feeds off of that fuse.
Would be nice if you could pick out all the splice connection
locations and disconnect each one to isolate the the fault.  

Personally, I would pull out all the door.trunk/hood switches first,
keep the wires from grounding, replace #15 and test from there.   If
any wiring or rewiring was done  - like aftermarket stuff- that would
be the second thing to disconnect.   The third thing is ground.  You
may have lost the ground on a subcircuit and this is backfeeding - or
not.  The fouth place to take a hard look is all the wire bundles from
the hole in the firewall to every tie down attachment point for
looseness  and abrasion.

You might also try using a breaker instead of a fuse.   Loosen up the
fuse block and on the back, pick out any wires (there may be two or
more) that  #15 fuse feeds.  With the breaker in place, touch all feed
wires to find the hot one.  If nothing else this will help to isolate
a subcircuit and a wiring diag should tell where the wire goes.

Not to insult anyone, but your customer has a glorified Studebaker
that was built by machinists.  All the parts that could be machined
are a work of art.   All the parts that could not be machined are an
afterthought.   The wiring was designed by a retarded machinist who
got the job of electrical engineer and who has no concept of  basic
wiring safety.   Mercedes pulls full unfused battery power into the
car on a #10 wire and then feeds everything off that wire.  

Sounds like this car is about due for a wiring harness fire, like so
many before it.    Don't let your customer drive around with this
problem uncorrected.  It may cost him the car.

This is a repairable problem, but it will cost more than 7.50$.

Pete

Mercedes - my daughter owns one, worked on many, less than impressed.