I thought I'd share a couple of pictures of a custom speaker installation I did for a friend and his 68 Camaro Convertible.
First, the front speaker installation. I used my plasma to cut a flat piece of steel out to house a couple of round Kicker 4" coaxial speakers. This piece mounts under the center of the dash under the dash pad, just like on the Mustang.
and then I made some custom enclosures to house the rear 5 1/4 speakers, and we put them in the interior quarters.

I know about the gap...we weren't quite done screwing in the panel.
I hope you don't take this is a sales pitch. I have no interest in this company or sales, I am only interested in passing along this information to the VMF members because I was so impressed with their offerings and prices.
How about this link, I use them for awesome name brand speakers. MBQuarts, component speakers, infinity kappa, kicker, JBL, Rockford Fosgate, individual speakers, and awesome prices

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MB Quart ESA 213 5-1/4" EXP Series 2-way Component System