Hi,
I am new to your forum and would like to know if it is possible to replace my old CD unit which has broken in my S 40 with a CD unit from a V 50. The only difference with the units is the plug system on the back, is it possible to get an adapter to make it work or do i have to find another CD unit.:

 

what are the model #s of the head units you are looking to swap out? You may need to check if the V50s was set up for a center channel speaker or not. Is your S40 a Gen1 or Gen2 (ie 2004.5 in the newer body style)? There are connectors available on eBay which you can splice in (you may have to search for the S40's radio connector pin out to do that). Also did you try to see if the new unit fits correctly into the dash on the s40? The gen 1s are compatible with the older "double din" radios going back to the 850s/V70s etc. I'm not sure about going from a gen 2 or a 2004.5+ model with the newer console design. (I dropped a head unit from a V70 into my S40 to get a CD player and it was a snap fit - cost me $50).