Audio Systems: Mod old stereo CD to accept MP3 player inputs


Question
I have an old Magnadyne LS7600CD stereo system in my RV http://www.magnadyne.com/catalog/part.cfm?PID=613460&CID=36884

My goal is to disable the CD player and instead input audio from my MP3 player. This has standard headphone out which of course will also work as "Aux In" for any rental car I have driven lately.

I opened the unit, cut the coax audio line between the CD board and the main board, and spliced in the red/white/black wires from an old headphone extension cord. I then plugged my MP3 player into the female end of the extension cord.

When I set the unit to CD it plays audio from the MP3 player! It sounds great, but very very quiet.

Not sure what the problem is. According to http://www.sweetwater.com/insync/headphone-outputs-used-line-outputs-for-line-le, a headphone output is nearly the same as a line output.

As an experiment I put a little Radio Shack "mini amplifier" in between. It puts 200 mW into a 16 ohm load, i.e. it is for driving a speaker. This did make the setup louder, but not loud enough. When I turned it up, the sound became very distorted. I assume this is because it is a high gain amp.

This mod is very close to working. Is there any chance the headphone extension cord is attenuating the signal? (It is a about 10 feet long.)

Or do I need a low gain amp? Any help will be greatly appreciated. Thank you.

Answer
I will put it all back to factory and do the aux input after the FM tuner, there is a am FM preamp and after that there is a preamp chip before the main amplifier ic chip you have to connect your new audio source after the second preamp chip and you have to turn off the power or ground on that chip that way there is no FM signal but your volume and eq controls will keep working

you can use mouser electronics or digikey to get a pdf of the chips with the info and what each pin are,
good luck