Sooth,
Maybe I didn't understand your question but maybe this will help, I have found over the years that tearing into the dash or doors is the scary part if you are not sure what you are doing, I know I have broke interior car parts before which of course are replacable but they do cost money. Before you pull the old deck out decide if you are putting in a single, 1 1/2 din or a double din unit back in and of course have it purchased, (Make sure it will fit in the dash area which Crutchfield or Best Buy can help with) You will need to purchase a wiring harness for your year of mustang, (about $20.00) Best Buy will probably have one, if not then an audio store will be able to help you, you will also need a dash kit sized for the size of head unit you are putting in, (also usually about $20.00)
http://www.crutchfield.com/ is also a place to find these items if Best Buy can't help, Your wiring harness allows you to just wire the harness to the back of the stereo you are putting in (using electrical twist on connectors) then it plugs into exsiting wire harness in dash, if you decide to try to do your own install when you get into the dash you will see what I'm talking about. You will also need to plug in the radio attenna, The dash kit will allow you to mount the stereo in the existing opening in your dash while also using some of the supplied hardware in the stereo you purchase. This very basic install uses all the existing speaker wire in your vehical and existing speakers and amps. A quality head unit will increase the quality of your music and allow you more control of the sound and imputs or outputs depending on the head unit you are using. Think ahead though and do some research for you may want to upgrade the rest of your system as you go so buy the right head unit now to upgrade later. If you want to truely increase the sound quality of your music then your next step will be speakers and maybe an amp.
the12volt's install bay - Mobile Electronics Forums
The above link is to a great sight that might help you understand everything from the basics of car audio up to designing your own system or building custom enclosures for subs or speakers. You will find people in this forum that can help you with any questions you have concerning car audio including installs in Mustangs. Good luck, have fun and I can also be a resource if you have any questions. I'm not a professional but I have done a couple of Mustangs now.
Terry
How do I replace a MACH 460 Head Unit? 
