Problem with shelves

[The Guardian]

Hi all,

Having finally bought the remaining speakers to fill in my 5.1 system, I decided to put up some shelves for my rears (which don't have mounts for brackets).

Got the shelves cut, fastened the braces on and went to install them into the wall... only problem is, the drywall anchors (self installing type) won't go through the drywall! Turns out that even though our house is really cheap, they apparently did use the metal-backed drywall in between units.

I really want the shelves in the corners if I can... so I have two options. First is there any kind of anchor I could use with metal backed drywall?

If that won't work I think I'll have to move the shelves over a bit and use a wall stud. Only problem is the studs aren't even to each side wall (one wall is an outer wall, the other is an inside wall) so the speakers would no longer be centered!