This should be the same for an S1 as it is for my early S2.
The method I used has been used by others. The current is so small and the wires never move that twist and tape seems to be acceptable. I would have soldered but I'm shit at it and probably would have ruined it
OK so here's how I did it:
Remove steering column cowl (2 Phillips and 2 torx screws)
Unbolt stack from underneath (2 x 10mm bolts)
Unplug wire to back of Stack. The ones you need to splice into are A2 (black) ,A3 (green) and A4 (white/black). These are the earth, ignition and tacho respectively. The blue clip is numbered so you can actually see which wire is which. This way up a is the top row and wire A1 is on the right hand side.
Strip insulation of said wires (I used a scalpel and still managed to cut through a strand of copper :tsk and wrap corresponding shift-i wire round it. It's quite fiddly as the wires are so close together and fairly tightly bound.
Cover joints in tape and bolt it all back together. Mounted the light at the very back of the binnacle pointing slighty upwards. I can see it over the top of the steering wheel.
Seems to reflex nicely like a HUD on the windscreen too. (y)
Seems to work on on the default sensitivity. One light comes on every 1000 rpm from 1-4000 so far. Didn't go any higher as I was sat in the garage and the engine was a bit cold. Can't for the life of me set up the lower and upper limits though. Didn't seem to work when I did what the manual says. Will have another go tomorrow.
I reckon from start to finish it was about 50 mins. Not bad as I'd never removed the binnacle before and the wires were a bit of a PITA to do as they are so small.