Start by checking thoroughly your battery and charging system, if it checks OK, maybe do the visor trick. If this fails, you might need a GT1 scanner. It might be GM3 module.
So start with the low-cost, easy to check stuff first.
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 M54, e53 N62 & Tribby