Temp guage not working

maybe thermostat or head gaskets gone.

Is both heater hoses hot? Could need radiator and heater core flush. Water pump propellers could be detoriated. I have seen this on vehicles before. Just some ideas to look at.

Do you smell coolant inside of the vehicle. If so then chances are your heater core is going.

Well the first question would be what does the temp gauge on the dash show after you have run the engine for 5 or 10 min ?
If the gauge is in the middle of the temp range the engine is getting hot and either the heater core is stopped up or you have a problem with the blend door under the dash.
If the temp gauge is staying around the cold mark the thermostat is not closing to warm up the engine.
I am assuming the radiator is full of coolant.

With blower on high and tempature to max heat check to see if you have good air flow in all mode selections (defrost, vent, floor etc.). If so and you still have no heat then check heater core hose going from engine to inside car to heater core with vehicle running and coolant tempature about half on guage. If both hoses are hot and coolant in radiator/overflow bottle is full then hot cold blend door is stuck in cold position. If coolant level is low you will get no heat, check coolant level first.

Am sure you have a thermostat stuck open, i suggest replace it and make sure the aantifreeze is in the correct level.

Having a blocked heater core will not make the engine over heat. You will just not get any of the heat from the fluid that would normally be pushed through the heater core because your core or cores are blocked (I think you have two cores if you have a rear heater.). The engine does not need your interior heater cores to keep cool. When you select "heat" on your console, either a valve opens up and allows fluid to move through the lines to your heater cores and or an air duct opens to the cores to blow air across them...

Did your heater work before your work?

Does the gauge stay pegged or does it fluctuate?

If your gauge now fluctuates after your work, it would indicate to me that there is air trapped in the engine or bubbles passing by. The air does not cool the engine block as well so the temp will rise quickly. It will then cool a bit as the coolant splashes by. If your gauge is staying hot and never moves, it could mean: you are missing a lot of coolant, trapped air near the temp sender, or your thermostat is not opening. A closed thermostat would make the gauge go all the way hot and stay hot. The thermostat can be checked with a pan of boiling water on the stove. When boiling, put the thermostat in the water and check to see if it opens. It is also important that the thermostat is installed in the correct direction. The temp sensor much be on the engine side. I am also assuming your pump shaft is not sheared and is still connected to the impeller and it is spinning as well. You can take your belt off and move it by hand to see if you feel any resistance. If you are sure the thermostat is opening and the pump circulating, I would then check and see if you have air in trapped in your system. I looked at my Uplander and noted a lot of hoses and metal pipes at a higher level than the filler neck of the radiator. Perhaps, when you filled your system back up from the radiator, that a lot of air got trapped in the upper part of the engine and the heater lines. I also saw that there were little brass valves near were the hoses connect near the pump and on the driver side. I would suggest running the engine and open and close the little brass check valve on the divers side to see if you have fluid or just air coming out. I wouldn

the hot water line from the engine goes through a valve that is controled by the temp control on your dash,older ones were cable control, the newer ones are usually vacuum controled,look for a vacuum leak at valve or dash control

the resister only changes the fan speed
if you have not heat
first check coolant level if it's down then you have a leak that will cause no heat if it's ok
then run the vehicle and make sure the engine temp gauge reads about half if not change the thermostat if the engine temp is ok
then grab the heater hoses see if they are hot then the heater core is plugged and if the heater hoses are hot
then there is a problem with the heater control head
hopr this helps

If the temp. gauge on the instrument panel is OK, test and eventually replace the temperature sender, and check the wiring to temperature sender.

Replacing the whole heater core is not a simple operation, test sender and wiring first.