I installed rear speakers in by box yesterday. I have the CDR 220 headunit and M490 amp. Somehow the radio code I got with the manuals did not work. Instead of going to the dealer, I went onto ebay and bought a decoding software for $3.71. And, it worked like a champ. It not only work with Becker but for a host of other brands. And its reusable. Here's the link it you want it:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270011518441&ssPageName=MERC_VIC_ReBay_Pr4_PcY_BIN_IT
The rear speakers do make a world of difference, especially when the top is down.