AC system on a 95 mustang

R134a . There can be several reasons it's not kicking on. There could be a leak in the system and it has discharged, or it could just be low like you say. Look underneath the compressor. Theres a wire that plugs in which controls the clutch solenoid. Check to make sure it's plugged in.

In the meantime, pick up a gauge for testing High/Low sysem pressure. Advance Auto carries it. I know you are supposed to test the system with the clutch on, but doing this will tell you whether or not if it is discharged ot even low. If it is just low, get one can of R134a freon and 1 can of ester oil recharge. The hose & valve that attaches the can to the low side port should come with the gauge. If not, then it may be a stand alone fitting. Can't remember, I bought mine years ago and I lost the valve for the can.