1999 VW Jetta, it overheats even after I replaced the thermostat and the plastic housing on the drivers side of the engine (it was cracked). I let it run with the res. tank open because I thought it might have air in the system, but nothing helped. Is there any other things to check?, the fans does not come on and there is no heat coming out the vents, also the lower radiator hose does not get warm.

Hello Kustomklean,

Eight (USA) domestic engine choices for this car.

The symptoms you are describing would
indicate a possible WATER-PUMP failure...

or

A heavily sludge condition (block) in the RADIATOR.

Before I would take ON that task...

I would do (or hire) a COLD PRESSURE test on the
vehicle to identify ALL cooling issues...
https://www.bing.com/search?q=1999+vw+jetta+water+pump+replacement&form=EDGEAR&qs=PF&cvid=76914cdfa4a141cdb6c35777d1deaca0&cc=US&setlang=en-US&PC=DCTE

And do consider of a real SHOP MANUAL: borrow/buy.

While the internet abounds with VERY entertaining
information... there is nothing BETTER than doing
the job correctly.

Stant Cold Pressure tester:
Amazon com Stant 12270 30 Pound Cooling System AndPressure Cap Tester...

Rockauto
http://www.rockauto.com/en/catalog/volkswagen,1999,jetta

Hope this helps

Carnac the Magnificent

Did you install the thermostat backwards? It could be air bound.jack the car up on the side the radiator cap is on and run it to try to purg out air.your new thermostat could be bad. Remove it completely and see what happens.as for fan not working check connections and relay.if it has air conditioning that works turn it on and the fans should come on.if not there is either no power or bad fan motor.check with a test light for power at fan motor connection.