That's what I finally did to track down leaks on my 300SD that were
driving me nuts.  Even with a hose, it can be difficult.   I could
spray the whole windshield area, heater air intake by the wipers,
corner pillar, door, roof, everything and no water would show up
inside.   Yet in even a light rain, in an hour water would be in the
heater blower and on the passenger floor.

I finally tried letting the water gently roll down the windshield,
instead of spraying it on.  Then water made it's way inside.  I had to
remove the blower to see where it was coming from.   With a mirror, I
could see it.  Turns out it was one of the holes for the plastic plugs
that hold in place the front edge of the chrome air intake grill around
the wipers.  The plastic plug was missing.   The water came through
that hole, then adhered to the underside of the body, traveling about 4
inchs across, then finally breaking lose and dropping down.

I also found a leak in the trunk from around the rear window seal and
even worse, from around the gas door lock.  The dummies at MB designed
the door lock with a vacuum actuated pin that goes in straight
horizontal, requiring a rubber seal to keep the water out.  If they had
just angled it downwars, water could never get in.

Good news is she's dry now!

You need a helper and may have to try for quite awhile to find it.  Try
to work one area thouroughly at a time.   Otherwise, even if it shows
up, without tearing everything apart inside, you won't know which area
it came from.