mustang seamless install

so my sister recently bought a new 2005 red mustang convertible which looks very similar to this one which isnt hers: http://norman.walsh.name/2005/01/19/images/Photo15.jpg . she has been wanting me to install her subs in the mustang for a few months now since she got rid of her gmc jimmy since...

alright so here is the problem i am facing right now. my sister recently purchased a used 05 convertable mustang. it has a stock stereo that is integrated into the dash http://mustangspeed.com/2010_mustang_stereo.jpg this isnt the same year obviously but it looks just like that where it is...