I tried to take as many pictures as i could during the process and i will try to answer any questions I can. I did this over two days due to drying, etc. I will try to make this step by step. I missed some pics in the initial process but you can reference the USB/AUX install pics below to help. Good luck and be patient.
Materials I used:
Plastic electrical dual gang box covers from home depot (they're blue, easy to find and a good thickness)
Aluminum angle 1/16 thick, also at home depot
JB Weld plastic epoxy
Bondo
painters tape
sandpaper
thick poster board for templates
Xacto knife
Spray paint
*You may want to source a new trim panel from a Salvage yard so you can put yours back later*
First thing is to remove your factory radio and measure distance between holes, I used a micrometer for this.Cut your aluminum angle to around 3" (+/- based on your model of navigation)
I drill holes to match the holes on the side of the pioneer. After attaching the pieces to the pioneer i used my micrometer to get the width and drill those holes. Then I test fit and used a dremel to makes the holes more of a slot and i could tweak it for later purposes.Now that you have that in the car you need the trim panel to fit. I used poster board to first make a template of the Pioneer size (approx 3.75 x 6.875) then i centered this over the factory location and taped it down... real well or it will move!!!
IT WONT FIT..
Next i cut the "wings" out of cardboard to fill the sides, easier to make templates first, then cut the plastic. I used those templates to cut out of the Blue plastic pieces I purchased from Local hardware. I did this with my dremel. I then covered the plastic wings with painters tape and stuck them on the trim panel. Now you can use the plastic epoxy from the back and get them nicely attached. I would try to get them to fit as tight as possible, it makes clean up later, much easier.
Once the epoxied had cured I cut out the top and bottom of the trim panel to give the height needed for the new unit. I used the same template that was already taped in place. Since i am a little over anal, i saved the pieces i cut out and used them for my side pieces with the blue plastic.
Now that your hole should be cut the size of your template you can clean it up and prep it for some bondo. Bondo is an art of a sort. If your unfamiliar i would apply it in thin layers to make finishing work/sanding easier. I recommend testing you bondo ability on something else first.
Once i had a good smooth surface and my trim panel looked clean, I prepped and painted it with the closest plastic bonding paint i could find to my interior. Let the paint cure at least 24 hours, trust me on this, it nicks easy when its not cured fully.
Here are some other pics of me doing the USB and AUX ports under the CIG lighter. This shows my template process a little better. Same general process as above, just different, smaller shapes!
I hope this helps and of course, I assume no responsibility for anything you may eff up in your attempt to follow my installation.