Looked into the posibility of retro fitting air con on a Renault Scenic I had a couple of years ago it was a no no. Think you may find the same problem with the b class
I think, because i had the same question about audio 20 after put audio 5, that everything that it has to do with panel layout its not possible to change afterwards...
It certainly is possible (my friend and I retrofitted it into his 1989 Honda Civic), but then again it was difficult and time consuming. We used the A/C parts from an Acura Integra (very similar design because Honda and Acura are the same company).