I discovered my rear demister doesn't work when the weather started getting colder. I checked the fuse (once I'd spent 20 minutes looking for a fuse box that doesn't exist in my model - thanks Haynes manual) - fine. The resistance across the heater element is ~5Ohms - seems reasonable. Can't measure a voltage on it with the switch on. The light on the switch doesn't light up. I sprayed it with contact cleaner - no joy.

I'm thinking it's probably the switch, but I don't know how to test it, since there are four wires coming out of it and I don't know what they are. The wiring diagram in the Haynes manual has no mention of the demister circuit, and I don't trust it anyway as it seems to be for an M49. 

As far as I can tell there's no relay, but lots of bits of conflicting information in various manuals and the internet.

Anyone got any clues? Cheers

According to the Handbook which is on here somewhere. Page 79 there's a 40A fuse in position F10 in the BSI, maybe red in colour. Most functions are controlled via relays in the BSI so I'd start there.

Yup, checked the 40A fuse F10 and it's fine.

Where are the relays in the BSI? Is that part of the fuse box in the compartment under the steering wheel where the above fuse is?

My Haynes manual mentions the BSI fusebox as a separate part containing 6 fuses (including the heated rear screen fuse, hence my previous confusion), but I think that refers to the M49.

The handbook has a list of fuses, but not relays.

Yeah, those fuses are part of the BSI, so far as I know there are relays and maybe more fuses within the BSI module to control certain equipment. I'm guessing that's how the rear screen is controlled but can't say for sure.

Im wondering if it could be a broken wire going to the tail gate?

Just been using my rear screen heater, there's a 'click' that comes from the BSI when I turn it on so am guessing there's a relay in there for it but no idea which one. If you have no light on the switch, I'd think your problem is somewhere between the BSI & the switch, perhaps the switch itself?

On my model

The BSI fuse box is below the fuse box under the steering wheel . The rear demister is fuse no 3 from the left @30amp. It also does the heated rear view mirror if you have one.

Thanks for that Cancunia, that confirms there is a relay somewhere then.

I suspected the tailgate wiring since it's an obvious weak spot, but yes the switch/wiring to the switch seems more likely. The switch looked in good nick - no obvious corrosion, but will investigate further. It's tricky because there's 4 wires and I've no idea what they are and they just disappear into a loom.

Thanks brodfather but I think that's the earlier model.

I had a look on Sedre and found the attached wiring diagram. It's for an M59 so should be correct but can't be 100% sure so no guarantee.

Bottom right shows the switch & LED with 4 wires, BSI is at the top and there looks to be a Multi Fuse in the box under the bonnet (PSF1).