Good advice above. When I need to deal with repairing electrical traces I first get the solder to stick to all the surfaces before I attempt to connect them. With the traces on the window you might have to CAREFULLY scrape the coating off to get to a solderable surface. You likely will have to solder in a short bare copper jumper wire to reconnect the lug and the trace.
I would follow this order:
1. Get solder attached to all the surfaces (lug, short jumper wire, trace).
2. Solder the short jumper wire to the lug.
3. Epoxy the lug back onto the glass. You might have to unbolt the hatch and flip it over to facilitate that letting gravity work for you. Don't break the glass!
4. Solder the jumper wire to the trace using the minimum of heat necessary.
5. Reinstall the wire on the lug carefully. I would support the lug with my finger to minimize stress when installing.
It seems odd that most cars have these window heaters and yet here in So. Cal. I don't think I have EVER used them.