Hi Colin and welcome to TM,
I used the same method as Roger and they work perfectly, I needed to buy two replacement light fittings which will take a double filament bulb, two switchback bulbs white/amber and in my case 2 resistors. I also fitted a relay (with a diode)
I did have a concern that when I switched the main lights on the speedo would jump to 40mph and then drop back, however when I asked BHM about this I was told that it isn't a problem and often happens. So no worries there then.
The cost of the parts came to less than £100 all in, so not too expensive a mod and very smart, and a safety feature as you get bright drl's.
All the credit must go to Roger
