How can I fix my heater / AC unit in my 1999 Buick Park Avenue ?

[DAVIDB]

I own a 1999 Buick Park Avenue, 3.8 L, V6, Base Model, the defrost work fine, but the " heater " part does not. When I drive out in the highway and I put the tempeature setting at say 74 or 75 degrees F it just sits there and does absolutley nothing - no heat !
Alot of times, there will be this " ticking / clicking " noise as if the blower motor is trying to do something.
The other day we were driving approx. 140 miles one way and it was 58 -60F outside temp and we wanted to 74 - 75 inside the car and I stopped and poped the hood of the engine and here all the various connections ( on the ends ) were all frosted up with white frost on the outside ! We had our mechanic replace the theromstat - but that did not change anything. The AC sometimes will work great and other times it will " roar " and the motor is " blowing " like crazy but no cold A/C coming out.
I just want to get this fixed for the wintertime so that my lovely wife is not freezing and the windows are not all frozen up. We live in PA.
Any suggestions of what might be happening and how to cure this issue ?
It's a 1999 Buick Park Avenue, 3.8 L, V6, Base Model
VIn # 1G4CW52K6X4645497
Thanks kindly!
Can I get any parts etc. from a reliable / reputabe auto junk yard ?
Or should I deal with brand-new parts ?

David

[pedro7of9]

problem 1...replacing thermostat for no reason...frost was a sign of incorrect "134R charge and or compressor problem
2 find a AC guy or go to [i hate saying it] dealer..mechanics actually have special tools/test equipment and most went to school to learn AC.