I'm attempting to fit an LED bulb into the rear light of my Matchless and know that with a bit of rewiring, a brake light switch and a resistor I can make a 'single filament' LED bulb work as a rear light and also a brake light. I know this has been touched upon before, but can someone explain in very small words or a wiring diagram/photographs just how I acheive this. For the purposes of this post can you all imagaine that I'm an idiot who knows nothing about electronics (as must be apparent by now, you won't be far wrong!!) 
Hi Bill, I'm glad you sorted it. A 60 Ohm resister I would have thought would make the rear light quite dim, but easy enough to get another resister. To be honest I lit a 4w bulb on a battery and then used different resisters on the 15W bulb until it looked about 4W
As for LEDs, they are alien technology
and I have no idea how they react to resisters etc. Easy enough to use a normal bulb 
email (option): horror@blueyonder.co.uk
Hi Bill,
I bought a 6V AC (So It is good for positive or negative ground) double filament LED bulb.
The amber ones are better for the tail light, as they do not lose much light when passing through the rear red lens,
And it shines very strong.
Also put one in the sidecar (White LED) and in the “Low” of the Headlamp.
When only the LEDs are on, the ammeter needle is not moving to the (-) AT ALL.
I did use normal 25/25W for the normal light and beam,
as LEDs will show you, but you cannot really see nothing when dark.
Got it from
http://www.superbrightleds.com/
MOT will usually not test if your No. plate is actually illuminated, but (if at all) only if the rear light lens/body das a white window on the bottom,
At least from my experience. Anyway, it’s not bright enough to see on day light, at the MOT test..
By the way,
If you use a bulb and a resistor, you actually draw current “spent” on the resistor and giving nothing but heat,
So better to use double filament of say- 5/21W, so you actually draw more than 5W only when breaking.
(For normal lamp, not LED that is)
Noam.
email (option): noam10@gmail.com
In the (currently) last post in the older thread, Horror has mentioned the need to use a diode. This would probably apply to a standard main switch but doesn't seem to be be necessary with the WD switch as the convoy lamp facilty means that the rear lamp has a feed which is independent from the headlamp.
The trouble with sending power to a switch on the wrong wire is you don't know where it's going to go. As these switches use a roller that clicks in place to the terminal next to it, it may have been possible to send power somewhere else on the bike with strange or adverse affects when the brake is applied, although the main power is turned off. Some bikes have Lucas switches with 2 rows of terminals and the internals are a mystery. The only way to be sure would be to put a meter on each terminal, with the switch in every position and try the brake. A diode is a safe guard against this and stops the current going back up to the switch.
As Rik says, no one has had any problems without one with a WD switch but I've put one on my 49 ES2 as a precaution.
email (option): horror@blueyonder.co.uk
Well, I'm glad it works, but I'm scratching my head as to why the resistor dimming method should work for an LED. Perhaps they have some sort of circuit board in the base of the bulb that pulses the current?
I dunno, but you learn stuff every day.
Kevin
There is something clever in there Kev as they work on both +ve and -ve earth. It looks like a standard bulb cap but there is probably more sophisticated electronics in there than on the rest of the bike put together.
Kevin
...and all for only 7.50 euro
http://www.wdbsa.nl/for_sale.htm
I can't imagine how little he buys them in for.
I don't pretend to know how it works chaps and I've no idea what else is in the bulb, all I know is it works fantastically. I had to play around with the resistors a bit and in the end I found that two 3v 68ohm and a 3v 46ohm soldered together before the switch did the trick.