I have a MK11 mondoe ghia 2.2tdci and had a problem with the air con and initially Ford quoted quite a high figure, well a very high figure, but I did some research and went to a few places. Along the way I learned that these units have a self diagnostic tool and for yours its this. Do the following with ignition on.
"At the climate control assembly, PRESS the "OFF" and "FOOTWELL" buttons simultaneously for exactly 2 seconds, then PRESS "AUTO" within 1.5 seconds. The on-board diagnosis which then starts lasts a few seconds. An animated display appears in the display of the climate control assembly during this time. Any faults found are displayed on both displays of the climate control assembly in the form of trouble codes.
Example: Left-hand display shows "90", right-hand display shows "B3", i.e. the trouble code read out is 90B3 = short in circuit for right-hand centre vents air outlet temperature sensor (short to ground)."
The mk4 air con is far more complex than the MK11 and an error code can stop it working. Give it a go and see what codes it throws up. There are a lot of things that can scupper air con, relays, fuses, broken fan, sensors, loose wire, wires corroding, leaks, and do on. I ended up going to a small service guy who was able to track it down to compressor not switching on but he could not find the reason. Ford have diagnostics but will charge. In the end I got ford to examine wiring and sensors, a loose wire in conector was the fault. It took them over an hour but bill was a lot less than original.