I'd take something from here. Depending on what connector you have behind your radio. Basically it emulates cd changer and adds bluetooth connectivity that way.
https://www.connects2.com/products/steering-wheel-control-interfaces/vehicle/Audi/A4
Hey. I have a B7 avant that I am going to do the 1din-2din conversion. I already have the same headunit. I also have the ashtray needed but would take your 2din cage if you still have it.
PM me