Each radio has a unique code. If you're lucky, the original owner may have kept the code somewhere in your owner's manual packet. Check there, including whether or not it may be hand-written in the owner's manual itself.
You usually need to reset the code when the power goes out in the car (dead battery, or someone unhooked the battery to do some work on the car). Someone mentioned a while ago about a site that you could go to to figure your code on your own...but never saw a follow up on that.
<span style='color:green'>1994 325i </span>- - - 75,000 mi.
No mods - - - very happy as is!
<span style='color:green'>1995 525i </span>- - - 154,000 mi.
No squeaks or rattles... unreal!