Random Water Sloshing around

I have a 1989 Mazda b2600 4x4. I have random water coming into passenger side. I cannot seem to find where it is coming from. Also after it rains when I drive around I can hear water sloshing around from behind the seat. Any help would be appreciated.

water in the rocker panels
drill a little hole in the back of the rocker to drain it
possibly have rotted rocker or floor up front, might want to pull the carpet back to have a look