I don't think Fiat ever fitted aircon to the 899.
Late 1999 they made a Seicento Suite, a special edition based on the 899 SX, but using an 1108 engine, standard ratio gearbox, not the close ratio one from the 1108 Sporting, and main point of the speical was aircon. Had a rush of bllod to the wallet and bought one brand new. 85,000 miles in three and a half years, poor thing.
Fiat used the 1108 with the aircon, as presumably adding it to the 899 would kill the performance. With the 1108 acceleration was gentle with the aircon pump running. When overtaking (notice in writing needed) I often prodded the aircon off, as if the pump kicked in mid-overtake it was like losing one cylinder.
Aircon was an option on the Sporting, but extremely rare as no-one really adds such expense to a cheap car.
On the Suite, the aircon pump was low down at the rearof the engine, below the alternator. Replacing the alternator would require de-gassing the aircon and removing the pump first. My alternator gave up at 85k, which prompted the upgrade to a Punto auto, rather than spend so much on the alternator repalcement.
I'd guess it is possible to fit aircon to the 899, but there are unlikely to be any standard parts to fit, and rare anyway. Bracket to fit pump to engine would be the biggest issue. Pipework, heater box including aircon unit, condenser rad for the front, pipework, etc, would all be same as the 1108, but still very rare, especially at this age.
Fitting the heater box means whole dash out, not too difficult, but time-consuming.
(My Suite suffered a shorted wire in the engine bay, melted all way back inside. Dash out to replace wire. £300 labour, less than £2 for parts. Done in a day at dealer, would have taken me longer. Cost was worth it for the convenience.)
Aircon 1108s were slightly more popular in Europe, but finding one and importing it is a lot of hassle. There are plenty of 2004 on Pandas around with aircon, which might be money better spent.
