does it rain alot where you are at/has it been raining?
if it has, cover your truck with something like a tarp or whatever.
then if it isn't or wont rain. get a hose and spray the truck down. By doing this you can see if you have a hole of some type that the water is going through to get into your car.
good luck.
You could have a heater core leak ... water circulates from the radiator and into the cabin area. This is used for heating.
I have an 89 MD+ 1 for spares they both had water leaks checked with a torch in corners under dash board had rust holes visible. removed guard to access problem for repairs.Check both sides seems to be a common problem
Check the front passenger floor pan toward the fire wall.
Floor at passenger side? Could be heater core needs replacing Other possibilities: door seal windshield seal
Are you running the air conditioner? If the drain line is plugged, it will leak water (condensation) onto the passenger side floor.
on the front of the engine behind water pump (water pump is bolted to it )
heater core may be leaking or hole in floor
Is it blood, oil, water, or urine?
There is a drain on the outside firewall that is clogged.
you could have a heater core leaking or your windshield is leaking and the water is running down behind the dash and onto the floor.
the cluth fan bolts right up to the water pump mounted on the block
bad heating core
Probably have a leak in the heater core>
Heater. Core