First, sell the audiovox. They suck. Then, buy a Stellar alarm. It is plug an play with all the factory hookups and a breeze to install.
Second, you can install an aftermarket deck no problem. You may have to bypass the amp, but it is not required.
Third, you dont need aftermarket speakers, but they are a much nicer addition.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>