you can add a sub in the trunk. they make a few custom boxes for the 05+ stangs. when i did mine i had it jus wired into the back speakers and mounted the amp on the back seat. the shaker 500 sounds great imo but i wanted something extra like you to now i love my sound. the fm transmitters are junk i would just change the head unit. heres a old pic of my speaker i got a jl audio speaker an amp now .