Rear demist wiring/switching challenge

Hi - I've tried to summarise further as I can see a lot of folk have taken the trouble to look but not been able to reply.
So hoping to give an easier read here will help someone assist.

The patient is a 1989 County 90 and the rear de-mist is not functioning.
The element itself is sound. There are no breaks from the terminals through the element.
Multimeter readings however show no volts or amps at the terminals.

At the front, the switch is activating the tell-tale light on the dash.
I have tried swapping both the relay AND the voltage-sensitive switch with new and 2nd hand items respectively.
Still no volts to the rear.

In your opinions, what would/should/could be my next stop?

Questions I'm asking myself are:-

1. Does the earth for the voltage switch or relay have a fault?
2. What's the best way to access the mount-points for these at the bulkhead? (Short of removing the entire dash).
3. Anyone know what circuit volts should make the switch trip and provide the relay earth? Perhaps my alternator isn't delivering the goods?
4. Where's the inline 10A fuse in-line between the relay and the rear element? Mr Haynes diagram says it's there but I haven't seen it.
5. Which fused circuit does the demist draw current from? The pictures below show the fuse-board layout and the circuit icons - none of which indicate the demist. The secondary fuses also don't appear to route the black-with-white stripe positive that feeds the demist.

Your attention, persistence and assistance appreciated....


FUSE LAYOUT (according to the cover)


MAIN FUSES


ADDITIONAL FUSES

Re-wrote this for a quicker read.
Hoping someone can help on any of the mystery items!

Think you need to

1) check the relay is putting out 12v to the load side - suspicion is it will be, particularly as you've swapped it out

2) Trace the wire from the relay to the heated window - chances are tha tis your fault. Check continuity from front to back - if no continuity, either trace until you find a bit that is melted, broken or chaffed, or just run a new wire to bypass

Are you doing your diagnostics with the engine running?

Thanks Gents.

@ Station House - Doesn't seem possible to get to the relay "out" without taking apart/removing the dash. Any tips on how to get there? However can get a finger or two to the relay in-situ and can't feel/hear any click.

@ slomofo - yes, engine running on test and have tried blipping the throttle. Did wonder if the alternator volts are insufficient to trip the VS switch. Any idea of best place to check circuit volts at idle (which circuit!?) and/or where to find the specification for the VS switch to find out what will trip it?

If relay is behind the speedo, just undo the 4 screws (2 on each side, 2 underneath) of the housing binacle and you should be able to pull it forward enough to reach in.

If more access needed, just remove the push pull blcok connectors (2 of them) for the warning lights and the speedo cable

I too suspect your problem lies between load side of relay and the glass - my wiring was knackered where it came out of the loom at the door hinge (in the plastic bendy pipe thing)

Years of flexing had done for it, so spliced in some new wire and hurrah!