2002 Ford Mustang car idle high when air is on

The air conditioning compressor puts more load on the engine when it is operating, the pcm compensates for the added load. Is your air condition system working properly? If your ac system is low on refidgerant and the compressor is not kicking in there would be no added draw on the engine causing a high idle.
I am not saying this is you problem, just one idea.
The high idle may also be caused by a vaccuum leak (ie: cracked or degraded vaccuum hose, leaking intake hasket or manifold, etc.)
let me know if these ideas help or if you have any other questions.
Dan