Took my 1993 VW Fox to a trusted mechanic for a new water pump & thermostat. He called me later to tell me the good/bad news. Good news - water pump & thermostat are in and cooling system ran at normal temp. Bad news - car caught fire after about 15 minutes of run time. Upon investigation, wiring harness under back seat is melted (which is why the car will not start - one of the melted wires is to the fuel pump). What happened?!?!?!

take it to a mechanic

I'm not taking sides in this. So what I am going to say is more or less neutral.
There is a big possibility you damaged the head gasket when the engine overheated last week. The dealership would have no way of knowing this if it was not yet leaking into the engine when they replaced the water pump.
Once the water pump leak was fixed, the system would be running under normal pressure which could finish off the weak head gasket.
You can not see a damaged head gasket without removing the cylinder head. And you have two heads with two gaskets.
So, IF the head gasket is now blown there is a good chance they did not know it last week. It could also be a cracked cyl head or leaking intake manifold gasket. Or all of the above.

You may want to check and see if the ECT (Engine Coolant Sensor) is faulty. It should drive the temperature gauge and if it's bad, it could be telling you that the engine is overheating when it is actually running at normal temprature.

Water pump is bad because overheats when engine is running at low speed, and good at high rev. The water pump impeller can not push the coolant with the required pressure.

it could be the temmperature sensor, however its sound like the thermostat is still bad, this happens on occasion where you get a brand new part that has a factory defect

good news / bad news

the good news is its a pretty simple job
the bad news is that some of the bolts have a tendancy to break making and easy job a pain in the ****

if you cannot find a shop that will do the job for less than $200 then seek out a high school or college that has an automotive program

most are always looking for work and "IF" they charge anything its CHEAP!!!

do not attempt to take apart fuel dist bad news pull off fuel inlet line spray dist full of carb cleaner try agin let me know has car been setting hey vote for me okl

Check ground wire from negative battery terminal to transmission housing. If wire is broken you probably fried your computer and hall sensor

I have had exactly the same thing happen to me. In the end it was a faulty water pump which was allowing water into the engine.

Try moving the fan blades by hand to see if the fan spins freely to see if the problem is frozen bearings in the fan motor. Replace the fan motor if it doesn't turn. If it spins freely disconnect the wires and hook the fan directly to the battery using a jumper. If it runs the problem is most likely the thermostatic switch. If it doesn't the fan motor is bad.