My idea was to simple use a variable resistor (potentiometer/pot) to attenuate the power getting to the dash lights. I started by removing the on/off switch by the left knee and measuring the current draw of the lights. It was 1.1A, so I made the assumption the resistance of all the dash lights 12.5 Ohm. To calculate the power and resistance value of the required POT I made a simple simulation in excel and looked at the power across the dash lights and POT for different values of POT resistance. This is what I found...
The max power going through the POT will be around 4W, and a good value of max resistance of the POT would be 20-25 Ohm, this would reduce dash power from around 14W flat our to 2W at max resistance. I looked around and high wattage POT's were not very common. At a local shop here in Austria www.conrad.at I found one suitable, part number 449831, a 10W 22 Ohm Potentiometer. The fitment was easy. The on/off  left knee switch has normal spade connectors so all I had to do was solder on a two leads capable of carrying over 1A and connect one male and one female spade connector.
I junked my old meta alarm and immobiliser for something more modern so the immobiliser socket has left me a spare hole in the steering column cover. The hole was a little too large for the POT shaft so I bought a few washers to hold it in place. The POT gets very hot when in use so make sure it’s positioned so it’s not touching the plastic of the steering cover.

 
Connecting to the existing on/off switch is easy as follows.

 
At first in the garage it appears I had chosen a POT with too much resistance as it was dimming too much, but out on the open road in at night it was perfect, at the maximum resistance you can just detect a faint glow.

I had a look at Maplins, RS components and farnell in the UK to find similar POTs but couldn’t find anything suitable. The ones on offer were either had a low power rating, or a very high power rating and were too big and very expensive. The POT I have is manufactured by TT Electronics, I emailed their sale office and was told the that Future Electronics, Arrow, TTI or Charcroft electronics would supply this component in the UK. I tried  Future Electronics and Charcroft electronics and never received any replies. The company I used in Austria also has an international outlet www.conrad-international.com and the part number is the same 449831. There appears to be a typo on the page though, the title for this part says 60 Watt and in the detailed description is says 10 Watt which is correct. Its 16.95€, but I’m not sure on the UK delivery.

Before I chose the POT I also considered a simple IC voltage regulator circuit, with a low power POT trimming the output voltage. This was going to be an easy solution but the max voltage output of such a circuit will always be around 3 volt down from the rail voltage, so the max power offered to the dash light may not be bright enough for city driving.

If any of you have swapped your lights out for LED's you will be using less current and will need a different value POT. If you need a hand choosing one I can help.

Anthony Mathers (kbear)