I have air con fitted to my Roadster and to be quite honest its a complete waste of money. Basically, all it is, is a cooling matrix in the air intake box under the bonnet. All the engine hot air gets sucked in as well and therefore the cool air comes through as luke warm.
Neil
If it is the 4/4 with the 1800 engine the same as my friend's '99 4/4 the air intake is much better isolated from the engine hot air as it has 2 separate intakes under the louvres.
On latter models the trick is to add foam or neoprene in such a way as to stop hot air from the engine bay entering the air intake.
We use our aircon often, obviously it is not terribly effective with the hood down but when the sun gets really too hot we put the hood up and the aircon on.
Borghi has fitted air conditioning to cars that did not have it so it is possible.