As your PM has interest to others that have also tried and failed using this method, I have posted the rpely to your PM here.
Thanks for your comment on my audio issue.
You say that it's not suitable for an MP3 input, as it's mono, but I'm wondering why that would make a difference?
(I'm a sound engineer by trade, but I do concert/event stuff & know next to nothing about car audio!)
I see no reason why I couldn't mono the signal from my player & put that into the input where the TM was.
Not all the pins on unit are connected, and the labels are often generic ones used across the range.
The mono TM input is of exceptionally low quality, in fact its so bad, I`m going to say it again, its exceptionally low quality, nope, it commands yet another, its exceptionally low quality and is also mono, meaning that you will not get ture represenation of the stereo sound from your MP3. Remember that these units were designed to drown out the squeks and rattles of the car, and this was many years before MP3 was a reality - and the bottom line is that they are simply not designed to accomodate MP3.
I`m also stunned that as a sound engineer that you do not realise the importance of stereo and mono !!
Yes, you could conceiveably try to join together your two MP3 channels, and if you didn`t blow the MP3 player audio outputs by short circuiting this, you would find that the sound quality is appauling, but then as a sound engineer you would already know this.
The only real viable option is the use of an FM modulator for the sound, but before doing anything else, I would spend a few hours trawling this forum for those who have also thought similar, and blown up, given up, and tried and tested many things before you. Thats the beauty of this forum, there is a lot of knowledge out there, but you need to do some reading.