in my 88 subaru GL, whenever i run the AC for a while. after i shut off the car and leave it for a period of time, when i come back, there is a puddle of water on the floor of the passenger side. it appears that there is a hose running up through the floorboard of that side. what do i need to do/replace?

Probably don't need to replace, just clear/clean. That rubber hose is the drain tube for the evaporator, and is supposed to carry off the condensate from the evap. What is likely happening is that the hose (or more likely the hard plastic nipple from the evaporator housing) is clogged and needs to be cleared, allowing condensate to back up until it fills the housing up to an opening/gap. If you put your hand on the housing after the AC has been running for a while, you might be able to feel the cold water through the housing. Remove the hose from the nipple, make sure the hose is clear, then do the same for the housing nipple. Be prepared for water coming out of the housing.