The trunk is fine, but don't mount it too close to the package shelf as this is the hottest part of the trunk. Behind the trim in most trucks is a brace which can be used too. Adequate power is another consideration.
Perhaps a better place is under the right seat. Problem is, most of the time you have to R&R the seat to do it right. The extra length of coax isn't anything to worry about.
Visit my web site and look in the Installation article, and in the Photo Gallery under Other Installs.