Thanks to superduper and my dad ( a retired tech teacher armed with his oscilloscope) I have successfully added a line in to my big ol' Helix!
Here is the left and right +ve connected to the tape function switch, the wires were crimped and soldered into female spade connectors that happened to be a nice snug fit onto the leafs of the switch!
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at the RCA socket they are soldered into place and the socket then grounded back to a point on the main PCB
here is the socket mounted on the back!
It worked very well but I'm not sure about the input level because I had to turn my MP3 player down to around half volume or it was simple too much for the speakers.
Anyhow! so far so good and his sucker is LOUD! ideal for a garage radio!