For god's sake, if you're going to tint your windows, don't use vinyl!!
Seriously, 6x9's aren't a great idea in the first place if you like to listen to high quality music. On the whole, they tend to be a bit "bling" at the expense of quailty. The biggest problem is that they play the music behind you.
I can't believe for a minute that that vinly is going to pass any high frequencies at all, so you'll be wasting half of the speaker's musical output. Acoustically tranparent cloth is very cheap, and does what it says on the tin.