98 Dodge Dakota Heater Not Working?

I recently changed out my heater core because it was leaking. I purchased an after market part which turned out to be slightly smaller than the original. After putting everything back together I didn't get the same heat as before. If I sit parked I get heat, but once I drive down the road, it cools off. Someone told me if I put it on max ac, it will recirculate the cab air, and then it works decently. If I switch it to defrost or any other setting, it starts to cool down.

It's as if I have too much air coming in and bypassing the core. The gauge on the dash says the temp is good and I have done everything possible to trouble shoot this (thermostat, bleed the coolant line, etc.). I also have a draft coming from the passenger side under the dash. What am I missing? I had plenty of heat before this all took place.

By Kent R