Also found this:
http://www.vw-resource.com/thermostat.html
Note: This isn't a hard job and shouldn't take you more than about an hour (at the most) to do. It is important to get it done before cold weather sets in, otherwise you'll never get warm air into the car. More importantly, proper operating temperature of the engine will not be maintained. This will shorten the life of the engine as well as lower the optimum amount of heat that could be used for warming up the cabin.
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If the thermostat has been removed by some enterprising PO, buy a new one. They're not very expensive, but you may have a hard time finding one of the original bellows design. The Mexican-style thermostat is an excellent replacement -- see below. Make sure you have the bolt and washer that fit to the bottom of the thermostat. You may have to buy a new thermostat bracket, too.
Note: If the thermostat is missing, there's a good chance the connecting rod and cooling vanes are missing, too. If that's the case, get yourself some new ones and install them in accordance with our Cooling Vanes/
Thermostat discussion.
Put the thermostat in a pot of water on your kitchen stove. Bring the water to a boil and watch to make sure that the thermostat expands. It should expand to at least 1-13/16". (If the thermostat does not expand, DO NOT PUT IT IN THE CAR! You'll have to buy another one.)
Block the two front wheels to prevent the car from rolling.
Raise the rear of the car and place jack stands under the torsion bars.
Crawl under the rear of the car armed with a large screwdriver, a 10mm wrench, and a 13mm wrench.
Locate the right heater box. Extending out from it toward the center of the car are two metal tabs, each of which has a screw in it. These attach to a metal plate (lower air duct plate) that has three captive nuts in it. You want to remove this plate.
Remove the 10mm bolts that hold the lower air duct plate in place. Stow the bolts and washers in a safe place. Note how the lower air duct plate sits so you can replace it the same way, then take it out.
Feel up between the cylinders for the connecting rod (about 1/8" diameter) coming down between them. This rod is threaded on the end so the thermostat can attach to it.
Right above where the lower air duct plate was there is a stud (possibly with a nut on it) sticking out of the side of the crankcase on the bottom of the engine, pointing towards the right heater box. If there is a nut, take the nut off of this stud and save it. If there isn't, you will need to find a replacement in your cache of extra nuts and bolts.
Attach the thermostat bracket onto the stud and replace the nut LOOSELY.
Place the thermostat into the bracket and screw it onto the connecting rod that just pokes down between the cylinders. This rod connects to the cooling vanes up in the fan shroud. As it moves up, the vanes open; as the thermostat contracts and it moves down, the vanes close.
With the thermostat screwed completely onto the connecting rod, move the thermostat up and down to make sure the connecting rod moves freely and doesn't get hung up on the cylinders.
Raise the thermostat all the way up, opening the cooling flaps. While holding the thermostat all the way up (flaps fully open), adjust the bracket so that the top of the thermostat just contacts the upper inside part of the bracket. Lock the bracket in this position by tightening the nut (13mm) that holds the bracket to the side of the engine.
Pull the thermostat (and connecting rod) down to the bottom inside of the bracket. This will close the cooling vanes. Bolt the thermostat onto the lower part of the bracket with the bolt and washer you took out of the old thermostat. Snug it up tightly.
Note: With the thermostat bolted to the bottom of the inside of the bracket, it only has one way to go when it is heated and expands, and that is upwards. As it expands upwards it pushes the connecting rod up and opens the cooling vanes. Cool, eh!?
Replace the lower air duct plate that goes under the thermostat and between the right heater box and the engine, with the four bolts you took out.
Note: The threads may be stripped where the bolt attaches to the engine and the metal around the hole in the tinware may bunged up. Do the best you can. Sometime when you're tinkering with the car on a Saturday you might want to wrap some Teflon tape around the bolt and try to secure it better. Not awfully important. Having that metal plate in place is VERY important -- it helps to make sure that the warmed air passes over the thermostat.
Jack the car back up to clear the jack stands. Remove the jack stands and carefully lower the car.
The engine is cold and the thermostat is contracted completely, which means the cooling vanes are closed. Before driving the car any distance, warm it up completely and then have a look at the thermostat -- it should be fully expanded (cooling vanes fully open) directing cooling air down over the engine.
Note: MAKE SURE that the thermostat expands! If not, you could burn up the engine. After driving it a ways you will find that the dipstick is hot, but you should be able to touch it and pull it out without burning yourself. If the dipstick is too hot to touch, the engine is not getting adequate cooling. If this ever happens, reverse the above steps and take the thermostat out. A working engine is more important that warm air in the car!
