X reg Scenic 1.9DCi Lighting problems. All sorts of things happening! Indicators go double speed when brake pedal pressed or rear fog lights operated. Indicators glow when side lights on. High level stop lights not working at all. Rear fog warning light on dash comes on when brake pedal operated or when indicators on , rear side light switch brings on brake lights and brakes lights only operate around half brightness etc etc etc? Have checked fuses and most connections, but can't find anything obvious. I assume an earth problem somewhere, but any help would be appreciated.

It would be easier to find clues to the fault using a circuit diagram.

There are slight differences how the fog lights are wired and generally are intended to be used only with dipped headlights but for exact operational details the vehicle handbook should be consulted.
You didn't mention what the rear lights including the rear fog lights are doing?

The fog lights usually derive the supply or the control for the circuit from the main lighting circuit so it would seem the fault is before the point in the lighting circuit of that connection.

I have the similar problem with my 2000 model. On mine I have narrowed it down to the rear light cluster on the right as you look at it from the rear (drivers side in the UK). Here's how I fix mine, although it does keep happening every now and again:
Remove the lights cover from inside the boot, it is held in place by a couple of plastic nuts.
Remove the strip of lights that are clipped into place.
There is an electrical connector block attached to the now removed light cluster. Undo this and the whole set of lights will be free.
Where the connector block fits to the light cluster clean all of the terminals. I use one of my wifes nail emery boards.
Make sure the female connectors on the connector block are all fairly snug so when put back together it is a tight fit.
Clean with electrical contact cleaner if you have any (not really necessary but I had some anyway).
Reconnect the block and test your lights, put back together.
I suspect it is an earth fault but I ran a separate earth to the unit but it didn't improve.
Other faults I noticed is the rear wiper operates when the front one only is on and I touch the brakes. Also when I lock the car when the indicators flash to show it is locked the rear high level brake lights flash as well. I was thinking of trying another light cluster, but 25 pounds from a breakers yard seems a little steep for a minor irritation.

Check on the brake pedal switch, sometimes its bad.
Source: All About Car cartechhome com

Check all ground connections and electrical paths for correct polarity..

check the rear brake adjustment and clean out all the brake dust.this can happen if the brakes are not adjusted properly as the misadjusted side will pull the brake shoes into the drum and act like the brake is on.if it was an abs problem or sympton the abs light should have come on to indicate a problem depending at what speed it happens.

Check & replace if necessary the brake pedal switch placed above pedals on the support.
If the problem still occurs you might access the ESP & ABS ECU (Electronic Control Unit) using diagnostic equipment connected to the data link connector of the car, in order to pull out all DTC (Diagnostic Trouble Codes) memored.
Then you will be able to replace bad piece(s) and fix the problem by erasing all DTC stocked inside ESP & ABS ECU & reset it.
I'm convinced that your brake pedal switch is faulty because the glow plug warning light went ON, along with the ESP & ABS. Just replace the brake pedal switch, see if rear stop lamps light when press the brake pedal, and test the car on the road.

sounds like a bad brake switch

Your problem is you have a bad connection somewhere like at parking lights/stoplights/rear lights/so what you do is remove all parking and rear lights then turn on indecators and check the lightsyou will find that one of them has a bad earth and when you put them back in the lights flash dimmly and thats the one with a bad connection or a bad earth going to that light.
Let me know if you still have a problem Ron

Sounds like an earth / ground wiring problem at the rear lighting units, when the flashers are on and you press the brake pedal the power is back earthing through the flashers. Check all earth/ground points at the rear lights.....Good luck hope this helps

Ok man, i don't if that helps you, but when i tried it in my 1989 peugeot 205.
Try this: put on the hazzard lights and that will test some of the brake light circuit.