I have a 2007 Nissan Frontier XL 4X4. I recently noticed that the passenger side floorboard is wet UNDER the floor mat. Could this be the air conditioner backing up?

Yes, or a hole in the bodywork under the mats. Does it do it all the time or only when you drive on wet roads?

The heater core is behind the dashboard and often central or on the passenger side.

Often the floor of the passenger side become wet if the drain pipe of the evaporator is:
- Bent or twisted.
- Clogged or,
- There is any obstruction in its routing.
The drain pipe is a rubber hose about 14 mm in diameter, located just under the evaporator (cooling coil) casing on the passenger side.
The most easiest way to clean the pipe is to take it out, wash it from inside with water or to blow a compressed air slowly through its distal end while leaving it in place under the body.
Note: Keep your eyes away while blowing the air.
Hope that it works.

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

seat? not floor board? seat could be a window seal broken or worn out .if its the floor i would say heater core.

Look underneath behind the passenger side front wheel. You might need to remove the plastic mud guard. It would be easier to see if you jacked it up and put it on jack stands.

your heater core(looks like a very small radiator) has a leak or is broken. Its under the dash between the driver and passenger. Check the hoses that run from the firewall and under the dash.

If this has happened since you started using the A/C it is probably the drain tube for the A/C. They get plugged and usually someone has to go under the car with a coathanger and open the tube up.

Clean out the drain tube .

The 05/04 Pacifica did not come with an elbow tube on the drain line for the AC drip pan. As you drive, the air flow into the tube pushes the condensate water back into the pan which is inside above the passenger floor board, the drain line is under the hood on the firewall. I spoke with the parts guy at the local Chrysler dealer, he sold me a drain tube for the 06 (~$10) and I installed it, wasn't too hard to do from the top if the engine is cold. The elbow slips on over the drain tube. Make sure to point the tube down. It only took one trip to start seeing water drip out of the tube and the floorboard started to dry up and almost three years later, the floorboard is still dry.

I believe the problem is a leak in your heater core. The water would run down the inside of the firewall or near it thus running under the mat.