Thanks to all who replied, but I think the project came to an end when I came across this post on an electronics forum


"My guess is that the speaker output - terminal is not ground, but is driven to some AC voltage out of phase with the + terminal. It's how most automotive amps (and other low $ items) work to maximize power out, given a single +DC voltage supply like you'd have with a battery (vs a + and - DC voltage supply that would be made from the mains AC line) . The speaker + and - outputs should not be connected to anything other than the speaker itself."

I suspect that the the radio I have has an amplifier as described in the quote, they say not to connect the speaker -ve to a common ground.

Dual voice coil speaker is still a possibility though .

Cheers

Paul