I'm not sure what an Av Mounting Tray is but I installed a Sony 10CD
(+MP3) Changer under the front passenger seat of my 1996 Legacy wagon.
All I did was :
-mark the place to put the changer with the seat in the usual position
(ie so you can't see it from the outside)
-remove the seat (easy, 4 bolts)
-put the changer on the marked spot you want it, mark the bolt holes on
the carpet (this Sony changer came with self-tapping metal screws)
-cut the carpet a cm around the bolt holes - carpet+underlayer is too
thick for the lengths of the screws
-drill pilot holes thru the floor pan with a drill with a carbide bit
(do 2 at a time) - the pan isn't very thick, you'll go thru - make sure
you're not near any wire or cable or anything under the carpet or bottom
of the pan - nothing near there in my Legacy
-screw the changer down with the metal screws into the pan...be sure to
cover the screw tips/holes on the underside of the pan with sealant so
you don't get any rust coming thru
-route the wires (usually 2) from the changer, under the door frame
bottom trim (pry it up carefully, there's a nice space under there for
wires), behind the passenger side kick panel (to the right of where your
feet are) and up behind the glove box (I used those zip ties to tie the
wires to some metal bars back there) and then into the back of your head
unit (your indash stereo). It requires you to remove the center dash
panel to get the stereo out.....
I've read some people don't bother with the screwing the changer down,
they just use big wide strips of industrial velcro to do it.....
hope it helps - here's some useful links too.....
pk
http://forums.nasioc.com/forums/showthread.php?s=&threadid=366982
http://www.installer.com/aux/
http://installdr.com/