Yeah installing the shelby kickers would be easy enough. You would need an aftermarket amp for the shelby take offs. They are like 200 watts at 2ohms or something. The stock Shaker500 amp would have under powered them. Also fabricating to fit in the doors...
But if you were going to add an amp to his system, I would just recoment getting a 10 or 12 inch sub for the trunk. I have a 10 inch sealed sub in mine, and its just a little louder than my shaker500 8's were. Plus its a small 12x12x12 box that fits right in the right corner with barely any trunk space lost. It sounds way better in sound quality to the stock 8's. A 12 would be good if he wants louder than the shaker500. Or a 10 inch in a ported box might do the trick.
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2011 Kona Blue MT82 V6 Mustang Pony Package 3:31 Gear Ratio, Interior Upgrade Pkg, Security & Shaker500/Sync
Mods (in order of installed): AM Bama SCT SF3 Tuner w/ 87 Hybrid Tune, Airaid CAI, Hurst Billet Plus 2 Short Throw Shifter, Lethal O/R H Pipe w/ Stock Exhaust, Blacked-Out Tints on Rear Windows, Full Ceramic Tint on Front Windows, Metra Touchscreen Dash Adaptor, Single DIN Head Unit, 6.5'' Component Speakers Fronts, Stock Rear Speakers, 10'' Sealed Subwoofer, 4ch. and 1ch Amplifier. LED licence plate lights, LED interior dome lights, FRPP/Dynamic shocks and struts, SR Performance lowering springs, GT500 Strut Mounts, BMR Adjustable Panhard Bar, BMR LCA's, BMR Relocation Brackets. Rear Seat Delete.