Fix your ground first. This isn't a car. When you attach to a bolt...you are attaching to just that. A bolt in fiberglass, with no electrical connection to the rest of the boat/electrical/motor.
Grab a flashlight, find the ground bar, and do it right. The 05 generation has a ground bar up under the side, in front of the drivers armrest.
PP is VERY sensitive to proper voltage, so don't take shortcuts. Find the proper ground source, and then see if your power wire works.
Not positive on malibu wiring, as its been a while.....but Purple on a key switch is often the "start" wire, not Ign power. So, my guess is once you fix your ground....it will only work when the key is in teh start postiion, cranking the boat,
Take your time, pull out a 12v. tester (or ****, a lightbulb with leads), and test your connections first. Make sure its good with the key on, and off with the key off.