The rear light, whilst stylish in my eyes was equipped for two festoon type bulbs, and with a 40 W E3HM dynamo this was going to be a problem. In my experience it is easy to get a bright, low current LED for the back but using an LED in the front is less desirable, so, aiming to use my power budget in the front, we need an LED for the rear.
Now, since the light we have is a bit of a pancake (pork pie? cheese?) shape compared to the 529 on the Square Four or the Wipac 'Sabrina' on the Bantam, we can't use a regular bulb holder. In this case I'm going to use an LED light board which I bought from CDRC recently.
To use this we will need to remove the festoon bulb holders. This is as easy as drilling out the rivets:
I don't want to clean any of this up - I like the patina of the old light and the corrosion will not affect function in any way in this case.
So here's a simple group of components - the lens, case, back plate and LED board:
Since the area the rear light fits in the number plate is quite badly damaged, there is limited opportunity for fixing holes. I am going to use an existing one, but since any fastener in this hole will be inaccessible after the light board goes in, we must make it captive. I'll use a small coach bolt, fitted into a square hole in the light unit and retained with cyanoacrylate.
The LED board sits on two brass posts retained with M3 screws, so we need a couple of holes for them. Use a blanking plug to close the spare hole and a grommet to protect the wires.
The cables need to be restrained somehow, so that an inadvertent tug on the wires does not pull on the solder joint to the LED board. To do this, we'll use a tie-wrap to retain the wires to one of the brass posts supporting the light board: