My heater up front blows cold. the heat in back works great.what is wrong on 2000 Chevrolet Tahoe

Ive replaced the actuator under the dash and both in cab air filters what else could be wrong

Asked by for the 2000 Chevrolet Tahoe
1 answer
I did not do anything as yet,cant find temperature door actuator or location cabin filters.

this is 2000 chevy tahoe 5.3.