There's something wrong with the S40 I just bought, When the radio is off, the back light on the display works fine,
but turning the radio on kills the back light and nothing happens when putting in a CD or cassette. I'm hoping I can somehow fix this, as I have no idea what the radio code is

 

Radio code should be the first 4 digit of the last 6 digit of the VIN.

 

never heard of the 4 digit rule from the VIN... I checked my 2000 S40 and the radio code is not related to the VIN in any way. So if you don't know the code a few suggestions - slide the radio out to see if a P/O has written on top of bottom of the radio, check to see if its written in the owners manual (four digit with numbers ranging 1-6). Otherwise use the VIN and ask a dealer to look it up (only good if its the original radio). I can sell you my S40's original with code - cassette/no CD... (no cd is why I dropped in a new unit). replacements are pretty easy to find on eBay, takes about 5 minutes to install - just by one with the code.

 

Never heard of it either.Didn't even read it from someone who heard it somewhere.Thats how my numbers are on my own car.