Thermostat??

I have recently noticed an issue with my 98 Jimmy...prior to yesterday it was blowing hot air on level 3 and 4 but then last night it was only blowing lukewarm air. There is no sweet smell in the cab and no moisture on the passenger floor so not thinking its a heater core but could it be a thermostat? I also haven't noticed any grinding or creaking noises from the motor or dash and no sounds of water that may signify air in the rad..but won't rule that out completely.

I park the truck outside at home but at work I'm in heated underground parking. So shifts from cold to warm environment fairly regularly. The temperature has dropped within the last day or so (-4ish) and that's when I noticed the change in what is blowing out. The air isn't cold and the windows don't fog but also doesn't heat up.

I have to change the blower motor resistor as that has been gone for awhile...could that be causing the problem? Also does anyone have detailed instructions on how to change that? Is it under the dash like in most other cars that I've worked on?

If it is the thermostat..where is it located in this thing? Is it an easy change for someone with only basic mechanical skills?

Thanks in advance for the help!

Spec:
1998 GMC Jimmy 4Dr 4X4 SLS Rocky Mountain Edition
4.3L Vortec Engine
Manual Climate Control
A/C with a leak that hasn't been corrected to date

The stat will be under upper hose where it goes to engine.

But first thing is to check coolant system for full of coolant in radiator.
Then check for what tempt the engine warms up to.

A good stat should warm the engine up to 195 degrees for good heat.

Post back engine running tempt.

Automatic or regular AC/heater controls?
If regular the resister will be right rear engine compartment on heater housing.

The temp runs no warmer than the mid. Just under the 210 mark. I had a cracked rad back in July and I replaced the rad and everything was working fine until yesterday. Have noticed that the temp is running slightly lower today- closer to the 155 mark than the 210 mark like it usually does. I know that it never runs over the mid line as thats what I've been watching for since the rad cracked.

It is regular AC/heater controls. No automatic controls.

Is the heater housing on the passenger side? Near the firewall? Sorry I'm a visual person so I need to know what to look for using other components as a guide. Thanks for the help.

Yes right side is on passanger side.
Trace the heater blower hot wire to it.
2 screws holding it in heater housing.
Right side is always on your right setting in drivers seat.
If tempt gauge is not coming up to 190-195 put a good brand 195 thermostat in it.
Let us know how it goes.