OK, I have been playing with the Mexico. I had about 50mm remaining of the umbilical cord, and I stripped it back, and then stripped the constituent wires. I found about 4 coaxes, and several other thin wires.
I powered the unit up and probed the outputs with an oscilloscope, and subsequently discovered that at least half the wires in the bundle don't go anywhere, and pulled out easly. I was left with the screen (ground), a thick +12v supply and two coaxes - left and right audio presumably. So in the end it was quite simple.
What I see on the audio lines with the oscilloscope is not what I expect, but admittedly I didn't have an aerial connected. So the next step is to give it an antenna, and perhaps put the audio into an amp and speakers so I can hear what's going on.
I fitted a new belt to the cassette mechanism, but it is still not engaging. More investigations needed.