I think what he is talking about is a 6-CD changer you can buy anywhere;
Kenwood, Sony, Alpine etc make them. There is a controller that mounts via
velcro in the cabin, and there is a converter that 'broadcasts' an FM
signal from the changer. You tune the converter to an unused frequency in
your area and then tune the radio to that frequency to listen to the
changer.

They also come with adapters for some radios that have Bus inputs and DIN
interfaces; these will plug into the more popular head units that use Bus
inputs and have integrated CD changer controls, such a Sony, units for
Ford are available; as far as your Yaris/Echo, you'll have to check at a
good aftermarket sound shop, or try online with some manufacturers. Toyota
radios USED to be supplied by Panasonic/National, Clarion and Fujitsu
Ten/Eclipse, so I would start there. Of course, my knowledge of Toyota
sound systems dates back 12-20 years ago. I don't know if these are still
current suppliers.