The CDRB20 IMHO is pointless.
When you select aux yeah it cuts out the CD, but from the people Ive spoken too, if the AUX is still playing when the CD is on then you hear both the CD and AUX audio together!

Now that struct me as being a little silly.
Needless to say that I didnt want to find out with my own money
So....
If you guys want to have a look at the schematic of the RB20, I stuck it up a couple of years ago, forgot the hit counter tho. Its a little over 1mb of jpeg, enjoy!