Scooterotica

Any ideas as to what can cause this. Spanish S2 Li All lights/horn work but put the brake light on and headlight and rear light go off. Earthed ok and tried a new brake switch but no joy. Any suggestions of what to try?

It would appear that wire from the brake switch to back light may be shorting to earth. The back light unit may also be the problem.

Try disconnecting the brake light wire in the rear light housing. If the lights still dim then it's the wire (or brake switch), if the lights are good have a look in the light unit.

I usually take another earth wire from the light housing and earth it on the engine, just to be on the safe side.

Good luck

Are you running original 6v points system or 12V electronic?

Is it an electronic or an original system?
Try taking the back light bulb out and try it - it might be a faulty bulb

yes, It's electronic.

Bad earth to rear light can do this but serious dimming of headlight is often caused by too much demand on the system, especially at low RPM... as when slowing and braking for corners. If this is the case, try a 25/25w headlight and a cheapo multi cluster LED rear, the type that is a straight swap for the standard bulb (Bosch bayonet fitting). This frees up power and helps keep the headlight brighter.

This isn't just a dimming of the lights, they go out. I'll try some of the suggestions thus far

^... mate's Indian restored GP, with dodgy quality rear light unit, was doing exactly the same... going out completely. It had me scratching my head for a while but a better earth from bulb holder to frame sorted it immediately.

had a brake light switch cause the same issue, was shorting to earth, so worth checking if you don't find a fault elsewhere

could it be a slight cut in the cable my light switch is on the gearchange side and every time I changed down the front light went off I eventually found that the gear change had cut into the cable could the brake pedal be slightly moving the wires?