I need to get to the key illumination wiring to wiring up other accessory lights, what's the best way to gain access to these wires?

Remove the steering column shroud. 3 (from memory) screws accessed from underneath. As it's a pain in the a** job with minimal clearance and the risk of breaking the plastics and you're only looking for acc lighting I'd advise going from either the footwell lights, cig lighter illumination or the T bar shifter light. All of these are MUCH easier to get at.