drilling holes in trunk (amplifier install)

so i was wondering if anyone could possibly tell me if it is safe to drill holes and put screws on the shelf type part of the trunk in a 91 grand marquis...i wanna make sure i'm not gonnna drill right into something

in this picture ( i know its of like a 2000 something model) but on the shelf type thing where the tire is sitting, thats the area im talkin about in my 91, it fits my amps for my new stereo perfect and i really dont have anywhere else to mount them.

http://grassrootsmotorsports.com/images/whatsnew/2007_mercury_gm_trunk.gif

maybe some pics of what is under that (or if its just hollow) would help me alot

thanks, J0n-

Your gas tank is just below where you want to drill. Be very careful.

ya when it stopped raining i crawled under the car and checked it out, it looks like i have a bit of clearance but someone told me they have used industrial velcro to keep sub boxes in place, and subs weigh a lot more so i am gonna use some of that. then take a few more precautions to make sure the amps don't fall off the shelf if they ever did come unvelcroed.