I used proper windscreen bonding glue. To say my last one was stuck to the roll cage is a minor understatement - when I removed it, it took me a couple of hours to carefully tease it away!
As for the wiring, be wary of the current draw - on Fords you can't run the heated screen until the charging circuit is active, such is the draw, and the worry of discharging the battery. Fit some hefty wire - on my daily driver the feed to the screen is through dedicated fused wires directly from the battery.
Incidentally, I'd try and get a heated screen relay from a mondeo / fiesta / escort (not a focus, mk3 mondeo, or mk6 fiesta), as they have a timer circuit in them.