Leaky Winshield

One of three things:

Leaking windshield or air plenum at the base of the windshield (hit it with
a hose to find out).

Leaking heater core under dash (if the fluid smells like antifreeze and/or
your antifreeze level is dropping).

Clogged drain hose from the bottom of the AC condenser box (when you run
your AC, does water drip out from the firewall in front of the passenger's
front seat onto the ground under the truck?  If not, the drain is clogged
and condensate is backing up and leaking into the cab).

Cheers - Jonathan

My S10 had the same problem once.  Finally found that there was an
unsealed seam in the right front wheelwell.  A garden hose there may
help you pinpoint it.  (I used RTV inside & out to seal it.)

Remember to pull the seats and carpet to dry everything out once you
solve the leak.  There ain't no other way to really dry it; it can
seem dry on top, and have a bucket of water underneath.

Good luck!