Originally Posted by 200xpdi
I am having the EXACT SAME issue with my '04. My headlights and interior lights will not turn on and my ABS, brake, and "security" lights are always on. I recently installed a Defi boost gauge and ever since then I've had issues. I am a complete airhead when it comes to the electrical system. Let me know what route you take to solve this and I will do the same if I figure it out first.
You kind of answered your own question on this one. Check and double check the gauge wiring - or better yet, take it all out. If the problem goes away, you know you that you tapped into something you weren't supposed to, and the gauge is now acting like shunt and putting power to a ground, wrong circuit, etc.