Mine have both come loose (left and right). Do I have to solder them back on to the window (where they were originally connected?, or could I use some sort of adhesive to attched.

Thanks Kelvin

Picture of rear window etc

When mine did this I used superglue, then conductive paint (for fixing the "bars" across the rear window heater. If you make sure you paint all the way round then there should be easily enough current capacity, but checking they don't get silly hot afterwards isn't a bad idea.

I've no idea if soldering would work, I was scared of localised heat cracking the window.

Electrically conductive glue.

Something like this http://www.frostfighter.com/

Bond the tab back on.