Just having a scout around for a solution for timing the heated front screen and found this.. http://www.vehicle-wiring-products.eu/VWP-onlinestore/relays/timers.php GLR10 looks ideal for hooking up to a momentary switch and giving a ten minute blast to the window. If it works half as well as the Ford screens then 10 mins will be more than enough.
Here's an option from Maplin, thankfully their pricing is WYSISYG and shipping is £2.99, though you'd have to factor in the price of relay(s) to be driven by the kit.