My passenger side floorboard is getting it a stopped up drain plug? on 1993 Nissan Pickup (D21)

I have had that happen in another vehicle but it may be the heater can I tell? If it is the heater core, does the dash need to come out to replace the heater cote?

Asked by for the 1993 Nissan Pickup (D21)
see if fluid tastes sweet if yes its coolant. hr job for a good tech. dash needs to come out to a certain point yes