For better or for worse the P-Bus is the only interconnect and includes the audio. But Pioneer sells a pair of adapters (CD-RB10 & CD-RB20) that feed from two RCA jacks onto the bus. The CD-RB10 is for when you just have a head unit and contains a single plug that goes into the head unit. The CD-RB20 has a cabled plug and a socket and goes between the head unit and the changer.

That's how things work in the normal world. But the Lexus techs tell me that the actual radio is inside a bulkhead in the rear of the car and all that is in the dash is a display shared with the climate controls, trip computer, etc.

I've ordered a CD-RB20 and will see what I see when I take the changer out of the dash.