Seamless smartphone integration
 

At the moment I've got a satnavless Symphony system, which has been serving me very well radio-wise. Having deinstalled the CD changer and a fairly ancient Nokia telephone (added by the previous owner I think), I've got quite a few connectors free:
https://www.the75andztclub.co.uk/for...ca7561aedc.jpg

My objective is to implement AUX, USB, and Bluetooth connectivity - primarily for phone audio purposes - while retaining the original/authentic ICE look and experience. I would like it to use the main car speakers, to be activated via the "TEL" button, and to follow the play/pause/previous/next buttons on the radio and the steering wheel properly. Perhaps in a perfect world pressing it repeatedly could cycle the AUX-USB-BT inputs...

Additionally, I have thought about replacing my Symphony with a High-Line unit for the more modern look and (maybe?) better compatibility with the aforementioned phone stuff, but the one sold on Rimmer Bros is more expensive than my 75 itself, so maybe not...

I imagine there are plenty of units for the phone side of what I'm planning - such as this one: https://amzn.eu/d/fgtv7xW - but I don't know anything about the Rover radio units, which of the connectors are responsible for the button control, and so on.

Would appreciate some advice!