the carputer will be no problem ... 90w only pulls 7.5 amps at 12 volts (yay! i can divide) ... and you won't run that thing at capacity.
And I wouldn't worry about the subs/amp either... especially with the new car. I don't know the specs for the jetta off the top of my head but i'm sure it will be fine. A 1f capacitor would be a good idea tho ... especially if you're into bass.
make sure you take lots of pics of your install. the wife has a jetta and maybe i can convince her to let me tear it apart if she sees some pics
