['LIVEWIRE?']

I don't have an easy answer to that problem, Duncan. A speaker level tp line level converter should in theory be OK, but bear in mind that you are connecting a mono radio's output to a stereo amplifier, so the output from the radio needs to be split between two inputs via resistors. Have you also taken note of the points raised by Leon Crampin in post #3 above i.e. that the radio may have an internal fault. As Leon says, the Frankfurts of that era have plenty of audio output to produce more than adequate volume and drive efficient 4 ohm speakers. If using two speakers, connect them in parallel to the RH pair of speaker sockets as viewed from the rear of the radio. With four speakers a fader control is, as I said, needed. Maybe it's a generational thing, but, to me, you are trying to find a 21st. Century to a solution, which, in the mid 20th. century, didn't exist. Radios of that type were fitted in all the makes & models of cars then available, using only the output facilities provide by the makers, and worked adequately that way.