You should be able to find the amplifier, its the big part with fins, heat sink
It will have 6 wires, 12v and ground, and 4 smaller audio wires, Left and Right channels, hijack it there
Extend the 4 wires from the "radio" to the headphone jack and then 4 wires back to amplifier
Actually 3 wires since audio ground(-) can be combined
You need the type of jack that disconnects the 3 "radio" wires when head phone cable is plugged in, so it should have 6 connections
It may only have 4 or 5, left + and right + IN, and left + and right + OUT, and you combine - out
So the radio wires are disconnected from amplifier when you plug in to the jack and head phone cable is now connnected to amp
Volume control would now be from the phone or player