This can be a bear of a job. Why don't you just ditch the DSP system (which isn't that great to boot), and go straight aftermarket nice double din with a decent amp & speakers? You will have all the bells & whistles, better sound and a much easier install process.
Looking for a DIY? Parts? Check this out, it might be your ticket
TMS underdrive pullies - Stewart WP - PSS9 - Beisan Vanos seals - Zimmerman cross-drilled & Akebono Euro - Deka 649 MF - 55w HID headlights - 35w HID foglights - Hualigan double din - ACS (rep) alu pedals - Euro central storage console - Breyton Magic Racing staggered wheels - M5 bumper - M5 steering wheel - Tint
Stable: e39, e53, e46 & Tribby