My AC on my 2000 Frontier is not blowing cold air, the compressor is running and the line(high pressure line) going from the switch into the firewall is cold but I put a thermostat into the air vent to see what the temp is and it is not getting cold, any ideals of what might be causing this? Oh when I unplug the thermo behind the glovebox the compressor shuts off and when I plug it back in the compressor comes on so I'm thinking that thermo is working properly, maybe its not..any help would be highly appreciated!

Might just need a charge. You'll need a gauge set to check to see if the Low and High side refrigerant line pressures are within spec.

R134a A/C charge kits aren't too expensive. I payed about $60 for mine. It has 2 gauges and a hose to connect a can of r134a refrigerant.

It was low on freon, I thought the compressor would cycle when its low but apparently not..Now I have to find where its leaking from

injecting a dye would be the easiest way...
but you knew that

When the coolant gets too low the compressor will not come on. When there's to much pressure in the system the compressor will cycle excessively.

I've experienced both first hand.