I thought the later ('series 2' with all synchro box) Series IV's had - Earth? But I stand corrected as I know Pruyter is pretty knowledgeable; on the Series cars in particular :-) G.
Well plenty of Radiomobile radios about for a few pounds, but only LW/MW which doesn't give much choice for listening. As you'll be negative earth it gives you more scope for a later FM radio or radio/cassette; even CD player, but I guess it depends how original you want to keep things? G.