Or...

Buy an HK Drive+Play and route the CD changer audio out to the input of the DP and then the output of the DP to the radio. As Long as there is a cd in the changer playing you will have sound.

The only other possible option that might work is the Sony adaptor that allows you to run multiple CD changers. It may provide the necessary data information to keep the cd changer port open if it simulates the same protocol.