should be listed in the owners manual somewhere. Another possibilty is that iit is on the back of the unit.

There is actually a decent chance that you will have to pull the radio out of the dash to get the code, but not because the code is actually on the back of the radio.  The dealer might require that you give them the serial number which is printed on a sticker on the back of the radio in order to look up the correct code.

DON'T JUST PUNCH IN CODES RANDOMLY!

Some factory stereos have firmware security devices that PERMANENTLY disable the radio if you punch in more than a certain number of wrong codes - 3 or 5...  it's not many.  Not sure if the 325's came with this security device or not.

When I coded the radio on my Miata, I just took it to the dealer and they looked it up by VIN, punched it in, and I was on my way.  Hopefully it's that easy for you!

The code can only be gotten from the dealer, unless you or the previous owner wrote it down. My car came with the code printed on a pair of credit card sized cards. If you have all the owners manual stuff they may be with it.
Otherwise you can try a coulple of things before driving anywhere: pull the radio for the serial # then call the dealer and ask for the code; good dealers will do tid for free if you supply the serial # and the VIN. Cal a few...if none will do it over the phone, you'll have to drive down and get themto verify you actually own the car, then they will give you the number. Greedy dealers who don't appreciate business may try to charge you for the help...just tell them no thanks and that your next new car won't be a BMW then leave.

As noted, if you enter the wrong code too many times the radio will be locked out; if not for ever, at least an hour.