Kev, if you have a 92 and aren't sure if its prewired, check the trunk. If it is there will be a small dot on the drivers side carpet that says its prewired.
<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>