From what i read in the previous threads and from the attached training file you can see that the Audio 20 outputs to Speaker Level outputs (which means directly to speakers) The only way i can see adding additional power and/or an input is to convert the Speaker Level outputs from the Audio 20 to Line Level outputs (like those required for amps)
With Line Level outputs, you can connect them into some sort of gateway like you mentioned which would have switchable inputs.
The problem with this setup is your Audio 20 has nothing to do with the volume when you connect your mp3 player to it.
Another possible way is using an inline FM modulator. There are some powerfull FM modulators that you can connect directly to the ANT on the back of the Audio 20. (you will need an addapter for the Euro plug > to typical FM ant. plug) I actually have these addapters laying around.
Then you can set the FM modulator to turn on from a switch, and connect the MP3 player to it as an imput. < this is the easiest way to try an AUX connection, and the least expensive. Total around $60 for all parts.
If the FM modulator works, then you can upgrade the power to the speakers using the above Line level conversion method.
Check out this PDF file (page 40) it will give you all the info on the Audio system in the R171
Train_audio_comm.Pdf.Zip by Bigupload.Com
(you have to wait till the counter goes down, then enter the letters and download)