You can install a PIE TOY03/PC-SON adapter which plugs directly into the back of the JBL radios (except for NAV) and then to a Sony CD changer, including their MP3 changers. You can use the steering wheel controls to control the changer. Here is a link to the adapter:
http://www.logjamelectronics.com/pietoy03pcson.html
I was originally planning on installing this adapter with my Sony CDX-757MX 10-disc MP3 changer. Here is a little info I got from PIE in regards to the adapter (ignore the part of combining with X3 adapter too):
QUESTION:
"Dear PIE, I am planning on purchasing a 2005 Toyota 4runner with the indash JBL 6-disc changer without NAV. I was thinking of adding a X3TOY so that I can add AUX inputs to the radio and control a Sony CDX-757MX 10-disc changer with TOY03/PC-SON connected to the X3. Will this combination work with the OEM radio? Logjam's website states the X3TOY is not compatible with indash 6-disc. IS this true? If this combination does work, will I be able to maintain use of steering wheel audio controls and rear seat audio?
ANSWER:
Yes, the TOY03/PC-SON will allow you to interface a Sony 6 or 10-Disc uni-link Changer (757MX) with the_OEM 6-disc radio in your vehicle._Pressing the CD or DISC button will toggle between indash 6-disc and 'external' CD Changer mode. You then have two options for AUX depending on how many_AUX inputs you need, one or three. The_part numbers_X3 and X3-TOY03 will allow you to add three AUX inputs, while the radio is in CD Changer mode you will be able to press a button (on small switching panel which flush-mounts to your dash or can be left inside a center console or glove box, included with X3 part) to switch between CD Changer (audio) and upto three AUX inputs. The part number SON-AUX_will allow you to switch_out of CD Changer (audio) into AUX audio input, while the radio is in CD Changer mode simply play the AUX device and the switching is done_automaticaly. Press_stop on the AUX device and the CD Changer audio will resume."
I abandoned this setup as I ordered an 06 with NAV. But it would be interesting to here feedback from anyone who is using this adapter to confirm operation.