Okay, followed all the good suggestions plus the vendor's... washed it twice, second time with a Mother's wax remover, clay barred the car, then washed each decal area with alcohol (vendor suggestion), toweled down between each cleaning with microfiber towels, so the car is cleaner than it's ever been since the day it was made.
Started my project tonight, and am totally exhausted, but thought I'd post one pic shot from the front of the car. Started at the back, did the easiest decal first, that was the gas cap, that was easy. Then the rear fenders, PITA, then the sports bar, getting better, then the headlights, and those look like a professional did them, then I made some hood flames just because I had extra and could modify them. All turned out great, still have front fenders and the windows to do tomorrow. I'd never suggest a decal virgin to do something like this as your first project - I'd say buy some cheap ones and cut them in different shapes, play around with putting them on your car, vertical and horizontal surfaces, and you'll find out how easy it is to screw them up pretty fast. They slide around almost too much when wet... anyway, by the time I was hitting my stride I felt like I knew what the heck I was doing. If I had it to do over again, I'd do the test decals as I suggest, because it takes just a bit of getting used to. Lining them up, and cutting around openings and seams is a PITA, but after doing the fourth one you're pretty much cruising...
So, here's a shot head on, too dark to take any others and I'm too tired. The blue at the top of the hood is painter's tape on new chrome windshield sprayers, and you can't really see the flames I put around those either.

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2006 PT GT vert: Getrag 5-speed, wind spoiler, billet shifter, 15" billet antenna, chrome brake handle, LED tail lights, modified air box, AMX TTAB & lower pipe, chrome curb trim, chrome headlight trim, PT sunvisor badges, chrome hood protector, chrome front & rear bumper addons, Mopar BOV...and so on...