As usual a bit more info is required, but its not rocket science.  Sun
roof leaks are common, I've never heard of a leak caused else where.
What year and model?, grab handles?, where are the wet areas - doors
roof, basically pull stuff up and trace back the wettness.

I don't really get what side your talking about, right and left of the
car is good enough for me.  I vaguely remember off side = drivers side
(ie not the curb side).

The front and rear windows are bonded, so leaks are v.rare.  Doors
usually pretty good.

We don't get really heavy rain or big hail storms here, but it might be
normal if the glass break or shock sensor is activated (most cars don't
have shock sensors AFAIK).

How do you know it is rain water (try tasting it), have you tried
lengthy squirting of the rear window (the cars squirty not with a hose)?
 The tube runs along the top right of the interior above the doors, it
actually freezes sometimes if cold enough and you don't use additive and
that could have split if the rubber is a bit hardened.

Does the car have grab handles on the roof, could these be leaking?

If none of the above yield anything you need to start taking bits off
and getting a assitant to hose the car (hose might be better than
pressure washer) while you look inside, and it might take a while to leak.

--
Tony

Just in front of the front door, down low is a provision for a foot air
vent.  Most cars just had a plastic plug in this hole (about 10cm x
10cm), I've seen a few of them broken.  This channel is connected to the
air intake grill at the bottom of the windshield, which is where the
water comes from.