I have a rear screen demister, controlled by a green knob on the dash. However, I'm not 100% sure it works. There haven't been many opportunities to test it, but I think there have been one or two, and it hasn't really done anything. The light goes green when ignition is on and it's pulled out, so there's a start. This morning I've tested the voltage at the wires/connectors. It's reading 12v at the wire going in on the (UK) driver's side. But on the connector coming out the other side it's reading 8-8.5v. Is that normal for there to be a voltage drop? Any other ways to test it?
What does it read across bothe the terminals. Should 12 volts I would have thought. Suggest there's resistance between the neg side and chassis assuming the switch is on the positive side. I would expect it to be.
You can but them new fairly cheap, I ran 12 volts through mine and check for what came out the other end, if something came out I assumed it must have continuity, still not had it steered up to check though
Turn it on and check that you've got 12V across one of the screen terminals and earth at the back of the van. Set the meter to low ohms, turn the switch off and measure the resistance across the screen terminals. Should be very low ohms.
Sounds like that with a reading of 8.5 volts on the negative side the earth connection is a bit weak. Probably it has snapped and the screen heater is grounding through the hinges. Or the earth connection is a corroded mess. It needs one side to be 12 volts, the other side to be 0 volts. Or looking at it another way - if you turn the heater off and measure the resistance to battery negative or another good ground from both sides of the heater you should be seeing a few ohms on the positive side and virtually nothing on the negative side. If you have a cheap multimeter the resistance of the meter leads if you touch them together will have to be subtracted from both readings to get the true resistance. My Maplins £5 cheapie reads about 2.5 ohms with the leads touched together. The ground for your heater should read under 1 ohm. Or if you were using my cheapie meter, it would read 3.5 ohms or less. So the real resistance would be 3.5-2.5 = 1 ohm. On my Fluke scopemeter that cost £200 on eBay when it was 20 years old... you can zero the reading by hitting a button when you touch the leads together first.
