I'm sure I found a couple of phono inputs/outputs! under the metal casing in the passenger footwell when I was routing my phone charging cable... might be worth having a feel around the usb socket area, my bluetooth module was velcro'd there too... from memory (not that good these days)... there was additional 12v power too!?
Might be worth investigating before unscrewing the rear panel.
Correct the bluetooth receiver is at the front near the volumee control. It feeds back via the phono leads to the amp.
