I found a multiwire plug coming from the radio board at the front of the radio, so I removed the plug and wired in a 3.5mm jack to the "L OUT", "R OUT" and "GND".

Passed it through and out the back, tinned the speaker cable wired and gave it all a good clean. Now I can plug my music into it in the Gym / Workshop.

I may add a cheap Bluetooth reciever and power it from the 5v supply that fed the radio board.