Now that's interesting, my heated rear (ahem) has some breaks in it.
a) What did you use
b) How do you find the break if not obvious - continuity maybe?
Funny you should ask. I'm just sending a refund request via Ebay. What an absolute crock of s%$t. It's a bit like sandtex. You peel the masking off really carefully when it's dry, and the line just comes off. I've a feeling that the masking should have come off once the line is painted and still wet, but that's not clear on the packaging. No adhesion at all to the paint and I can't imagine it staying on when the glass when I windowlene it. I'll get more success with the pack that uses a stencil you take off as soon as you've painted it on wet. Unfortunately Halfords have boxes and boxes of it, but the the paint has either gone off and evaporated, or people have taken them back after using them (well I do live in Preston). I used a tester, but every single break it showed corresponded to the line missing (so what's missing using the naked eye is what needs bridging).