Yes you are correct! But get a test light, not a test pen. Ground the test light and look for power, ign and you're pretty much set. It's very very easy - seriously.
Only thing is when cutting and joining wires, make sure you do it properly. Twist clean copper wires, bend and wrap right with shrink wrap or solder the wires nicely. I made a mistake of just twisting the wires and carelessly wrapping them in wire tape, it popped out a lot. Didn't help my suspension was hard and car bounced a lot.
---------- Post added at 07:40 AM ---------- 6 hour anti-bump limit - Previous post was at 07:39 AM ----------
OH btw.. the myvi will need a new faceplate or whatever you call that. Unless your Myvi is using the double din player? Mine's the stupid integrated one.