What do you use to attach the rear view mirror to the glass?
 
The old one was held on with big gobs of silicone but the mirror was busted, so it all had to come off. I cleaned up the glass and tried one of those little mirror kits with the wipe and some glue. The little block went on ok but fell off once the mirror was attached. I'm guessing the silicone left a film on the glass and nothing can stick to it now.
 
Should I just use more silicone? Or maybe araldite? Or is there some other glue that might work? I need to get it there for Sunday.

Araldite

NO NO NO!!
 
Go to your nearest autobarn store and get a Loctite or permatex mirror adhesive kit.  They are worth about $12 and will do the job properly.
 
The key is that you have stick just the steel bit of the mirror mount to the glass and then leave it for 12 hours or so before putting the mirror on.  If you dont do this they will fall off again in no time

Permatex, that's the one I tried. Just did the little metal bit and left it about 24 hours. All looked ok till I screwed the mirror on and then as it twisted a bit it popped off again. I'll go get another kit and try again, maybe a different brand this time.

I've used 24hr araldite lots of times and it stays on indefinitely. Trick is the glass and the metal tab to be extra clean with acetone (nail varnish remover) , mix araldite and put just enough on the metal , place in the right place the correct way up. Using tape to hold in place for at least 24-36hrs it should be there for life of window..

Thanks for the tips guys. I've had another go at it this morning, just putting the metal tab on, and taped it down. Fingers crossed it stays on there for tomorrow!