99 Malibu passenger side floorboard is wet could it be the heater core

No,What year,make and model is your vehicle Carol,when the heater core leaks,it will be the driver side floor board that gets wet,are you using your A,C,Theirs a hose on the passenger side that the A.C. drains to the outside ground,could of came loose,it's on the passenger side.

There could be a number of issues at work here. First, and most important is to address the source of the wet floorboard on the passenger side. Get you fingers wet and then rub your fingers together. Does the wetness feel a bit more slippery than just water? Smell it. Does it have a bit of a sweet smell? If so, this could be coolant leaking from the heater core into the passenger compartment. Slow leaks will take a while to notice in the coolant level. If this is the case, replacement of the heater core will be necessary. The wetness could also be just rainwater coming in from an open firewall plug, damaged door seal, or perhaps windshield seal, where rust or a poorly installed replacement could result in leakage. Now about the heat. First, is the engine coolant reaching operating temperature? Run the vehicle for at least ten minutes. Then feel the upper and lower radiator hoses. Both should be hot. If not, you may have a thermostat issue that is keeping the coolant from getting warm enough to produce sufficient heat. If the hoses are hot, then check both the supply and return heater hoses to the heater core. They should both be hot as well. If not, then a clogged heater core is usually the culprit. You may be able to get lucky and back flush the core, but most times replacement is required. If everything to this point has checked out OK, then it is likely that the blend door in the heater control box is inoperative and will not divert the cabin air through the heater core. This is hard to determine as it is all under the dashboard and sometimes difficult, if not impossible to see. Check the control head vacuum lines for cracks and breaks. Look at the vacuum actuators and ensure they are operating and that their linkages are connected as move freely. Repair or replace any problems found.

your heater core is bad they leak when they go bad youll have to replace its under/behind the dash

Yes

this should be the heater core,is the pipe itsself bad or is it just the hose needing replaced?does heat work,is the passengers floorboard wet with coolant?could be the hose bad or the heater core bad,some cars have whats called a heater control valve right there at the firewall that the hoses hook up to,it could be bad,if hoses just hook up to two pipes sticking out of firewall,check hoses first,then look in passengers floor to see if its wet,heater core is inexspensive but costly to have replaced.

Your heater core has developed a leak - the wetness on the passenger side carpet is engine coolant
from a heater core leak, or a leak in one of the two hoses connected to the back of the heater core.
Your strategy: disassemble dash down to heater core, determine whether you'll need a new heater
core or new hose (or new hose clamp), then replace the offending part.

Yes is it a sweet smell, is there wetness on passenger side floor does window get foggy. then yes heater core.

the water could have just traveled. check the heater core.it might be bad.

blown heater core

your heater core (a small radiator) in the heaterbox is leaking