If for heated front screen it will have a serious big wire going to it.
The purple one taped up is indeed air con, if you note down the relay wire colours then look at the wiring diagram around the fusebox you will see what does what.
The ford handbook lists relays for ABS(20 amp fuse,green), pump(abs) with 30amp fuse(violet), Fuel Pump on fi, if not this one is used for ecu on carb versions(yes they are rare).