Best thing to do is have the car running and swith on the rear demister Switch, have a kettle of boilling water and steam the rear window! (Ffs don't burn yourself in the process), use a blck felt tip to mark to point to point where its broken and use some cleaner I.e petrol, parafin, afterhave just to clean up the broken area! Mask under the lines and above the lines so basically when you pant the lines it won't look botched up!! Nice and neat, let to dry and then re-test! You should find the paint in the auto factors shop or definetly should find in halfords!! The pen is a special conductive pen that dries! You maybe able to get it in paint brush format that maybe better as long as you mask the galss away from the broken tracks as said!! Its easy to do, the only thing that may take time is masking it carefully!
J