From a set of Passat wiring drawings I used when fitting my Climatronic, the terminals associated with the 4 pin plug on the FCM are:
T4/4 12V Feed from battery
T4/3 Signal from F18 the radiator thermo switch
T4/2 Fan speed 2
T4/1 Fan speed 3
The fuses are:
S78 5A fuse for the coolant pump
S108 20A fuse for fan stage 1 and a/c magnetic coupling (although to me it appears that stage 1 is fed directly from the thermo switch which may then suggest this fuse is stage 2….hypothesis only!).
S107 50A fuse for fan stage 2 & 3
So if the 20A fuse keeps blowing, is there a chance you have a short to ground on the a/c magnetic coupling wiring from when you removed it? The magnetic clutch feed is on the 10 pin plug T10/10…or it is on mine which replicates the Passat at least. Or is the short on one of the fan stages, 1 or 2 dependent upon which stage fuse S108 feeds.