As far as i am aware, non sat nav cars DO NOT have the wiring for the naviagtion already in the loom.
It is however possible for the sat nav unit to be installed in a non satnav car, BUT the wiring would need to included, a few friends of mine have done this.
Bought the FULL unit from a salvage yard with the wiring, when i looked it had been cut out of the original loom by the guys at the salvage place, its in place and works fine cost him £1000
Not bad !! :D