I know this is a very old thread but as this seems to be only source of information for fitting an auxiliary belt to a 1.4 MPI AQW Fabia with air conditioning, I'd put in my two pence worth. I have spent hours for Googling for advise on this but there is very little information and as @opentoideas mentioned, the Haynes manual is pretty much useless for vehicles with air conditioning.
I have just completed mine this morning as the old one had come off the pulleys and was a little worse for wear.
As the OP mentioned, there is a special tool for this, which I found is as rare as rocking horse ****!
My solution was to use a large set on mole grips to lever the tensioner pulley as using my 10" adjustable spanner kept slipping off. Also the adjustable would only pull back as far as the light
fitting, my mole grips passed under the top edge of this and allowed a little more slack in the belt which was the key for me. I finally managed to pass the belt over the coolant pump pulley (C) pictured, which is very conveniently located under the top engine mount for ease of access 😆
As my belt have already come off, finding the belt routing was very helpful. Also once you have got the belt on, check that it is on all the pulleys correctly. I took a bit off fiddling to get the belt central over the coolant pump pulley and the tensioner.
Also the 7Zap website is an invaluable source of information for most makes of cars, https://skoda.7zap.com/en/cz/fabia/fab/.
I hope this helps someone else with this issue.