I've had what I thought was an issue with my Stack ST200 Tacho's wiring. As soon as the ignition is on the back lights been permanently lit. As there are only 3 wire's +12v, 0v and ECU tacho driver there isn't a separate feed for the light.

I've emailed Stack to question the back light being on and they have said that's correct. The ST200 tacho has/is a permanently lit gauge.

I now have to drive around with 1 gauge permanently lit or my side lights on all the time to light the other gauges. Ok it's not a major issue but it is a little annoying, maybe I'm a bit OCD.

Nope - that would annoy me too!

dismantle/reassembly possible?

I hate to say it, but that should have been obvious if only 3 wires.... how would it know your lights are on otherwise?
But yes that would irritate the hell out of me too!

You could try 213 Performance in Bicester, they may be able to modify the gauge. They were able to repair my Elise Stack dash when no one else would touch it.

Simple solution may be install a dimming pot?

Connect the others up so they are permanently lit too...?