I've been thinking about adding npp to my car for a little while, but here is my thought for an easy install. Take a 12v vacuum pump, like this one:
https://www.sparkfun.com/products/10398 .
With that, you would be able to control by switch. It would have no on or off effect by engine load, but work more like electric cutouts. Quiet when you want, loud when you want.
Anyone tried anything similar? Not sure how well it would open and close..