Step 2: Problem: No Heat At All
If the voltage is fine, the problem is somewhere in the wiring or at the grid. Check the connections from the wiring harness to the grid. It’s easy for the terminals at the grid to become damaged. Generally, the tab that’s attached to the glass breaks off, leaving you with a dangling wire and no way to reattach it. You have two repair options here: soldering and gluing.
If you know how to solder and have a high-capacity soldering iron or gun, solder the tab back on. It may take a third hand to hold the tab against the grid while you solder it. There’s usually a metal strip laid on the glass under the silk-screening. Clean the surfaces with alcohol and use 60-40 rosin-core solder. Work fast, because excess heat may crack the glass.
If you aren’t confident about your soldering skills, or aren’t ready to take a chance on cracking an expensive piece of glass, there’s another way. The dealership and most auto parts stores can sell you a special electrically conductive epoxy to bond the tab back on. If it’s wintertime, you’ll need to work in a heated garage, and have the vehicle inside long enough for it to warm up to at least 65˚ F.
Again, clean the area with alcohol. Mask the glass with tape to keep from getting epoxy smeared on it. Mix up a sparing amount of epoxy and hardener. Put some epoxy on the tab, and use an ice pick to hold it in place for the 10 minutes or so it will take for the epoxy to harden. You can use a wooden stick or the end of your dampened finger to smear the epoxy within a minute or two of application to improve the cosmetics of the repair. Although the epoxy will set up rapidly, don’t attempt to reattach the wiring until it’s had 24 hours at 65˚ F or more to cure and achieve its full strength. The repair will never be as strong as the original wire, so you’ll need to be particularly careful not to damage it in the future.