1) to remove the lock - push down on the steering lock plate and you will see a small, split, wire ring attached to the rod which actually does the turning. There are special tools for this, but you can either a) push it down, or if it's stuck b) hit it with a hammer to loosen it up (sometimes they get corrosion if they've ever been outside). To get the wire ring out, I use a 90* pick and pry it out. Caution - it is not easy.
This is an example of the tool http://www.tooltopia.com/lisle-57340...FeQCQAodJm3gRw note how it holds the ring down by pushing on around the parameter of the lock-ring?
Once you get the wire-ring out, the lock ring will slide up (there is a spring under it). Don't worry, I've yet to ever have one pop off - even when you get the wire bit off; it's a tight fit so you have to work it off.
Once you get that off, the signal switch is next bit that comes off (3 screws) - then you'll know what the wires are which control it.
I can say, with certainty, however that the purple wire is not a signal wire - on every GM car I've ever dealt with (50s-2010) the purple wire is the wire which energizes the starter solenoid.