rear demister probs

[sadler20k]

im currently havin a problem with my rear dimister on my 318is 1996 (P reg) an it is as follows; when i got the car the button was missing and i used to have to push the switch in using something small well since then the switch has stoped working all together and i have desided to wire up my own i have all ready put the switch in but do not know where to find the correct wires to put to the switch an also the colour coding of the wires, if any1 can help it wud b much appreciated.

Cheers Daz