So it started out when you turn the **** to cool, it only work turned all the way either cool, same when going to hot.
Now it will not go to cool, heat is on all the time.
I know it must be a vacuum issue, but where to start looking ?

 

start by downloading the 01 service manual. locate the blend door somewhere around the bottom of the hvac plenum. see about removing the dash bezel to expose the temperature dial, and see if you can see how it controls the blend, vac or elec. sorry, i don't know, but i'd guess its electric.

 

You can bypass the heater core to get relief from the constant heat in the mean time too!

 

The blend door should be operated by an electric motor on a 99. The motor could simply be bad or the switch on the dash itself , but this is more common. http://www.heatertreater.net/

 

Thanks guys, Got it fixed
It was the blend door interposer.
Got a new one from the local Stealer, was $14
Was going to order the Heater Treater but wanted it fixed today.
Took all of 30 minutes to replace, hardest part was cutting the carpet pad back to get to the back screw.

 

Yeah, that one can be a real bear. Glad ya got it fixed.