Peugeot 307 51 reg 5 disc Blaupunkt CD player?

After replacing the 5-disc Blaupunkt CD player (with a second-hand unit from E-bay) the radio won't switch to the replacement CD player although it will still switch between the radio and tape player satisfactorily.
Power to the CD player seems to be OK and it accepts and ejects the CDs OK.
Any ideas?

GC
December 2012