Vincent
the easy solution, is as Geoff suggests, revert to positive earth - there are few real issues with this.
remember that the generator is polarised, and you will need to repolarise the generator if you do this (i.e. it is not quite as simple as changing the battery terminals, but may well be simpler and safer than messing with HT supplies)
the early valve radios all had vibrator power supplies - effectively an electromechanical DC-DC converter, so to make the radio work on negative earth you need to swap the polarity of the output transformer from the vibrator
DO NOT DO THIS UNLESS YOU KNOW WHAT YOU ARE DOING - the output is high voltage and you (may or) may not live to regret a mistake
the only thing the radio actually uses 12v for is the valve heaters and the lights (and they don't care what the polarity is)
exchanging the vibrator for a solid state DC-DC converter is commonly done because modern solid state DC-DC converters are more reliable than vibrators, you wire them according to the polarity that you need - it is not necessary to change the vibrator to a DC-DC converter to change polarity
does the radio work (if you take it out of the car and power it up with the correct polarity)? if it does, then changing polarity is straightforward (see above), if it doesn't, then the radio needs somewhat more detailed problem solving (I suspect the latter, I know your car)
regards
Julian
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