Thanks a lot for all your help!

I think the bigger sub is no solution, too - like tanstaafl said.
I already thought about a way how to place the sub so that it can fire directly to the back of the boot.
Another problem I have is that there is a metal panel under the boot (parcel shelf). But there are some holes (for the standard-speakers) in it. I'll first try it that way. But I'll have to change the sub's chassis because it is to large to "stand" under the boot. Maybe I'll build a new one instead of destroying the old one.

When I'm finished I'll report you! Thanks again!

TeeMcBee