i know the outside is neg just because of years of playing with these adaptors, ad also just to be certain, i tested another one.

not sure why no curernt out of tran but never mind, yes cut wires close

also, managed to put it in car, all works fine, sounds okish, well good yer, but taken it out for now as ive taken all the crap out for the mot tomrow and then itll go back in on thursday

Good news (even if you relied on risky assumptions - I know for a fact that not all DC plugs use the same polarity!)

Anyways, good luck with the MOT. Where are you planning to fit the speakers?

not sure yet,
just off topic, do they check axel bolts as mine are a tad, erm clunky