[VX Wagon] Mounting 6x9 Enclosures - Where to Drill? Anything I can Hit?

[TheDmac]

I was thinking about mounting 6x9 enclosures on top of the wheel arches in a VX wagon. I was just wondering if there is anything I should be aware of in terms things hiding under or behind where I am planning on mounting the brackets. The bracket on the side closest to the tail gate has to go into the wall, as the arch area is soft and doesn't seem to be anything under there. The brackets on the side closest to the seats is going into the arch, as it's solid there and the wall is not. I've included photos showing where the brackets are going.

Side closest to to tail gate:

Side closest to the rear seats: