91 Cutlass Ciera getting warm air only

All of the hoses look fine, the radiator is okay, and when I put the vent, heater, or defroster on, even after the car is completely warmed up, I only get warm air, at best. The colder it is outside, the less heat in the air I get from the vents. And we have had temps in the below zeroes recently, so I've kind of needed the heat. My husband says it sounds like the thermostat, so I bought the thermostat, gasket, and Chilton book, which is completely useless. From what I can see, I have to remove the cruise control vacuum modulator, air cleaner assembly, throttle body, and radiator hose before I can reach the thermostat location. Could this be right, taking all of that apart? I can't find any kind of diagrams on the internet that match what my engine looks like, and the Chilton book is a joke. The pictures is shows for example don't come close to my engine. I just don't want to take all of that apart if I'm not even in the right area. Could someone help? Thanks!!!

What engine is in the car?
A few items to check.
coolant level in the overflow bottle, there are two heater hoses that go through the firewall into the heater core. After the motor has been running a while they should both be the same temperature. If one is much hotter than the other that means the core is partially blocked. If it is blocked use a razor blade to slit the ends and peel away the hoses. Use a hose to back flush the core. Remove the hoses carefully, you don't want to put to much stress on the core. After you do this you will need to run the motor with the radiator cap off and the car facing slightly uphill to get the air out of the cooling system.

I don't know if this helps, but it's a 6 cylinder 3.3 engine. Coolant levels and mix are okay, and the hoses are getting hot. The temp guage barely moves, and the radiator fan won't kick in. I thought maybe the fan motor was blown, but funny thing is, after the car was warmed up, I switched over to air conditioning and let it run for a few minutes. The fan finally kicked in, and the guage moved up to about 200.

We haven't smelled any antifreeze, and there isn't any indication of a leak into the passenger compartment, or onto the ground, so I'm pretty sure it's not the heater core. But my hubby still feels it's the thermostat. With his work schedule, he wouldn't have a chance to take it apart to see. I just need to know, before I start unscrewing things, if I have to take all that stuff apart to even get to the thermostat. If I know that's the correct route to take, I'll get busy, but don't want to disassemble half my engine if it's not right. Thanks!

I think you are confusing the thermostat replacement between the 4 and 6 cylinder motor. The thermostat housing on the 6 cyl motor is on the right side top where the upper radiator hose goes on the engine. Looking at the motor while standing in front of the car the housing will be on the left side top of the engine attached to the hose. It should only have two screws holding it on. Some motors have a sensor screwed into the housing but you might not have to remove it to get at the thermostat. Do not over tighten the screw holding on the housing or you will never get them off again without drilling them out.
good luck.