RR Classic heater fan, A/C problem

on my Classic the heater fan works fine blows hot and cold air through the top vents but (i dont use it at this time of year just a niggle i would like to fix) when i turn the A/C on the vacumn builds up as normal but the fan stops blowing completly, someone mentioned at some point that it could be a relay, would this be the general opinion on the fault and does anyone know where the relays for the A/C are its a 1989 3.5 EFI, someone before said the position of the A/C components can vary on the earlier ones that had it, i dont know if mine would fall into that group or not.

The relay is no 7 counting from the right as you look at the fusebox. The fusebox is under the bonnet fixed to the front of the slam panel on the passenger side. Swap one of the relays of the same size and see if it then works.