You would know if you had the Mach system. It says it EVERYWHERE! If you keep the same speakers and just upgrade the headunit, you will get a louder system. The aftermarket stereos put out more watts than the factory unit. Whenever you do the swap make sure you wiring everything good! Don't do a cheap and easy job of connecting wires. DO IT RIGHT! The previous owners on both Mustangs I have owned did SH**TY jobs on the wiring causing my dad and I all sorts of hell!
2000 Mustang GT!!!
FRPP Intake manifold, Flowmaster 40's with larger pipes and tips (Nice deep tone).
"I want to own a shop where people can come and not get completely ripped off" - Ludacris (Fast Five)