When the heater is on my windshield gets fogged up

sounds like the heater core is leaking into the car. this will usually always fog up the windshield and or leak on the floor under the dash. But, it may just be a minor leak to begin with.

Nothing wet under the dash or on the carpet.

it may be only a small leak at this point, as you turn your heater on coolant flows threw the heater core and you get the fog up effect. Honda's do it all the time also. same situation.