The dealer will know. I don't think there is any additional wiring. Are the 7 additional buttons just time realted, i.e. trip time, distance measuring? I have the 18, but I don't know what the 11 button consists of.
<span style='font-size:8pt;line-height:100%'><span style='color:blue'>1998 BMW 323is Coupe, Black
18" M3 Hyper Silver Rims
2 12" Rockford Fosgate Subs; Sealed
1.0 Farad Capacitor
800 Watt 2-Way Rockford Fosgate Amplifier</span></span>