Just got a 06 c70 and am very happy with it, apart from the following problem...

When I put the 3 cd cartridge into the CD player, the audio remain in radio mode and the cartridge doesn't seem to lock in place properly. According to the manual, it should automatically invoke the CD player when you put the cartridge in. Pressing the 3cd button doesn't make any difference either.

Would be very grateful for any suggestions ...

 

If you feel up to the challenge, then pull out radio and take a look inside. Unscrew top cover and verify that the small flat ribbon cable is connected to the cd laser and the other end to the board. Other than that you will need a technician to repair the unit or swap it out for a working radio.
BTW, 05 is the last listed year for the Gen 1 C70, then they went to a hardtop convertible with the multi changer being a music bank setup and not a cartridge style, good luck.