94 heater core replaced, still has fogging

I replaced my heater core in my 1994 GT 5.0 Mustang a few weeks ago. Ever since defroster still not working right. When driving defroster works about half, but real problem is when car is run then sits (a hour or two) windshield fogs up in middle about a foot wide (won't defrost for long time). Only 2 things i can think of is 1. heater core bad. or 2. There is leftover antifreeze left in heater box. My antifreeze level is still full since I filled it back up, so is seems I'm not losing coolant. Please help if you have any suggestions or have heard of this problem.:banghead: Hope I don't have to take this heater core out, What a pain!

Heater box drain maybe clogged. May still have moisture in the vent tubes.
Drain tube for condensation from the A/C maybe clogged.

I will take a look at that. Thanks.