96 cutlass supreme sl heat no work when cold

the heater in my car doesn't seem to like the cold weather. when it is cold in the mornings the heat doesn't get hot. Most of the time it is colder inside the car with the heat on than it is outside. But in the daytime when it heats up outside the heater will chase you out of the car. I have replaced two thermostats already and was wondering if anyone else has had this problem and what they did to fix it. thanks in advance.

Welcome to AF.

Depending on the thermostat temprature range and the outside temps it can take a while to warm up the coolant and heater.

Thier are two different thermostats you could use or have installed, a 180 deg and a 195 deg thermostat.
If the lower one(180) was used your car may not heat up adequetly to reach or blow higher temps from the heater.

If your heater core is partialy clogged it wont heat up properly either, and that's what I'd suspect has happend.

I'd try a cooling system flush and see if that doesnt help, if it hasnt been done already.