This new Sony radio has RCA line outs and amplifier enable control wire. So its compatible with the BMW amplifier.
No, (as you now know) the RCA outputs will not drive the stock amp's inputs.
The stock amp however is more than an amp. (well actually it is a booster amp which prevents any benefit from the fitment of a high powered head unit)
High powered head units are **** anyway, so it doesn't matter.
To get good sound you need to use the low level outputs (RCA connectors) to connect the head unit to a decent (so long as it is reasonably heavy) amplifier. Why BMW didn't do this in the first place is a complete mystery, they got so much right about this car (espeically the amazing 10-speaker sound system) it is amazing that they went and used high-level connections and a booster amp to power it. But hey, maybe Engineering had to give in to Marketing somewhere...
The BMW Booster amp is more than a booster amp. It is also a crossover which is why it connects to each of the 10 speakers individually through one of its connectors. It even has a special tuning module to adjust it to suit the accoustic characterisitics of each different interior. This is the part worth keeping.
First thing to do is remove the amp and check to see if it is possible to retain the crossover section: If so, the upper section (incorporating the heat sink) will obviously unscrew from the box- when you open it up you will be able to see that it will as the booster amp is a separate PCB at 90 degrees to the main PCB which is the crossover section. If it isn't possible all the booster and crossover components are mounted on the one circuit board with the power transistors screwed to the one-piece casing... so there is no unplugging of the booster module itself.
Now the point of doing this would be to install your new, additional amp using the low level inputs from the head unit- wiring this new amp's outputs to the cars crossover (booster removed) and thus simply retaining all speakers, wiring and the perfect crossover points for your car's cabin.
If your amp is not this and you are handy enough to replace power transistors, it is easily possible to retrofit an old 535 amp (which you can definatley remove the booster from) and chuck the old amp. You just have to check the connectors as some pins are slightly different (the remote on connection) between your model and the 535 one, but the connectors are physically the same.
Oh yes, and as a bonus you can even use the wiring in the car (what the head unit's speaker level connections used to use to run to the old amp) instead of routing a new shielded RCA lead. The remote on can come from the stock wiring to the crossover as will no longer be necessary to power the crossover unit. The only new wires you need to run will be the power and ground for the new amp and enough speaker wire to run from the new amp to the crossover inputs witinin the trunk.
And before you ask the stock crossover and speaker system can handle ooodles of power. It will sound 3x as good with a decent power amp and 10x as good if you add a subwoofer. (Hint- get a cheap 5-channel amp and mount it under the parcel shelf). It will be much louder (without distortion) than you would possibly imagine before doing it. I can easily deafen myself if I start driving my car regularly.
Nick
By now you must be feeling that this is pretty hard work... but stick with the program, you can make this really worth doing.