I've got one in my closet somewhere. I boxed it up after I took it out of my car...
The classifieds might have some for sale too, check them out.
You wont have the full Shaker 500 sound without the door subs though, and I can't guarantee that the speakers that are in your car right now are the same as the ones that come with the Shaker. But then again, I CAN guarantee you that aftermarket speakers are better than the Shaker ones anyway.

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Black 2005 Mustang v6, 5 speed, IUP, EUP and Leather.
Razzi 1/4 window scoops, Mustang GT Sway Bars, Flowmaster American Thunder Single Exhaust, Clear Front Turn Signal Lenses, Demolet CAI, X-cal2 91 tune, Hurst Comp/Plus Shifter, FRPP T-Lock, Ford 3.73 gears