89 trooper, fan works but no heat

I wanted to know if anyone has fixed this problem themselves before. Any ideas on the most common parts to go-the thermostat or the heater coil getting clogged? There's also a heater switch for the water line but since it does warm up a little bit it doesn't seem like it would be that because some heat gets through. The temp gauge seems to take longer than it should to move as well, but no problems mechanically in any other way with the truck.

I have some fully detailed manuals but they don;t indicate how difficult the labor might be. Doesn;t look too hard but the antifreeze has to be drained even to try replacing the thermostat so it's going to be a long night if that part doesn;t work. Doesn;t seem to be any electrical problems at all. I can probably get the front end into a garage to work on it but a whole heated garage isn't available right now.

The blower works fine and if it's running for a long while there is a little heat. This happened of course during our first big snow storm this year and I don;t want to be hit really hard by having to take it in during the storm.

Any ideas on how much it's probably wise to replace on the first go? if there are any particularly tough or tricky steps to watch? It is an 89 with a little over 100k miles but is in otherwise pretty good shape. No problems starting, idling etc.

Any hints from anyone who's done this would be most appreciated. Best to replace thermostat and heater coil together? Possibly the water heater switch and I'm wrong thinking it's something else? Any known tough spots getting any parts off? Tricks to help me?

Thanks a lot!
DHorner