Do you mean the night-time illumination bulbs that would come on with the sidelights? If its them then it sounds like the tail light is getting backfed from the brake light. Do you have an aftermarket LED tail light of some description or is it all standard? First I'd check that the brake light is coming on when you brake, if it is then check through all the connections. If its standard check if the brake light is coming on as bright as it should, it could be a bad earth and the electrikery is trying to get to ground via the instrument lights. If its an aftermarket LED jobbo the easiest way to stop it would to fit a diode to the tailight, in series. You can get them for buttons from Maplins. If it is that I'll find out the type I used and mail it up John
My pleasure to assist. Check to see if the sidelights are coming too, I reckon they will be. If its standard then in all probability its a bad connection or earth. If the brake light isn't as bright as it should be its almost a cert that the tailight earth is bad and the circuit is trying to find earth through the sidelight circuit. Check the connections and tailight earths as a starter. Bit like you see some not so well maintained old cars when indicating and the indicator and stop lamps flash alternately, similar sort of thing. With LEDs it can be worse as they can operate over a wider voltage range (they are really current dependent) so it all the lights function but wrongly if you get my drift. Be interested to know how you get on, if its not that then post away. There are a lot on here way more knowledgeable than me, I'm really a closet Guzzi man John
Brian, you need to check the integrity of the wiring harness that runs on the under side of the seat for any pinching, the wires sometimes get damaged under the seat pads, where the main harness lead pivots with the seat, at the seat catch or at the point where the wires enter the rear light cluster. Next separate the three pin connector on the harness fixed to the underside of the seat from the main loom, check condition and security of the 3 pins and also that there is no water stuck in there anywhere. Next open up the rear light cluster (under side of the seat two screws) and check that the bulbs are seated and there is no water hanging around. Finally for now, the rear light cluster earths to the main loom via the injection system three way plug (the big 3 way one close to the injection and fuel pump relays) separate this and check the condition and security of the pins, it is unlikely that there is a problem here but it does not hurt to check it. If you find nothing untoward you probably have a broken/damaged earth wire somewhere between the ecu three way plug and the light cluster. You can confirm this by either checking it from the light cluster to the ecu plug (pin B on the ecu side of the connector) with a meter or temporarily connecting a loose wire between a known good earth (battery negative post for example) direct to the rear light bulb holder. If the latter clears the problem it confirms the earth continuity is the problem and gives you an idea of how to bypass it.
