Keep the air con fan and the 2 main cooling fans seperately switched, do NOT join them.
With the fan rewire and it's new relay the wire that goes from the fan switch in the radiator is the one you want to hook into for a fan switch over ride, so assuming it is still switched positively from the original fan switch simply take a live to a over ride switch, then out the switch to the relay that the wire from the rad switch goes to.
30 is battery
85 is earth
86 is switch(fan switch in this case)(live)
87 is output
So hook into 86 to switch it manually from a over ride switch, remembering to switch off the over ride when you leave the car, or get a flat battery.
tabetha