Originally Posted by Thrush

Autoglass don't make glass. They buy from (nearly) all the same places as the rest of the replacement trade. What brand is the glass? Pilkington?

This is probably a stupid question, but you are testing the screen works with the engine running, right? Because it won't do anything with the engine off and just position-1 ignition.

If you're getting power all the way to it, including the switch, but it still won't work, then the chances are the elements are broken rendering it useless. They are thin fine wires, kinda like bulb filaments, and the screen has two "banks" (a left and a right) so if an element is broken, that bank stops working.

Which Focus is it? Mk1 or 2? If Mk1, what year? Mk1's had two different heated screens fitted - early one had the terminals at both bottom corners (each has two wires - 1 to power, in a connector, and the other an earth, bolted to the bodywork) and the later ones had a single connector bottom middle, with an earth strap that bolts to the body behind the front interior light. Mk2's have terminals at both bottom corners (power, in connectors) and an earth strap under the interior light.

Check all the terminals for water ingress - it's very common they have not been put back properly (located I mean, not connected) which allows water to get inside them and short them out. The mk2's suffer from the foil strap on the drivers side being ripped off by the wiper mech when fitters don't put them back correctly. The passenger side is susceptible for getting water in the connector if not put back right. Both mk's will probably have another earth wire behind the light, so check this is fitted correctly - sometimes if they're put back in the wrong order, it can stop them working (from memory, it should be screen earth strap against the roof, then the secondary earth (no idea what it's for, probably the light) between the screen earth and nut)

Also, check the mini blade 10amp fuse that makes the interior light work. This can often blow and sometimes, with seemingly no pattern, stop the heated screen work...