Toyota 2004 Coaster 12 v converter

Factory fitted, the wiring from the back of radio disappears under the dash.

If you are certain it is factory, then I would expect there to be a factory fuse for the radio somewhere in the fuse box. (This would be a reasonable indicator you are correct). Find the fuse (by removing one fuse at a time if the fuses are not identified) & check the voltage, just in case it is 24v (indicative of the possibility of a step down transformer between fuse & radio ... or a 24v radio). If there is a 24v to 12v converter ‘box’ like I had it shouldn’t be too hard to find with a bit of wire tracing detective work, but if it is just a tiny step down transformer specifically for the radio that would make life difficult. I suppose you have already checked the voltage at the radio? If you are correct about it being ‘factory’ it is as good a reason as any to get what will always be useful to have - a workshop manual for your model. Good luck.

The original radio in 24volt Coasters are 24 volt. After market radios, being mostly 12 volt require a 24v to 12v convertor of some description. Two Coasters I've had, have had a small unit screwed to the back of the particular radio. These units supplied just enough 12 volt power to run the cassette/radio in question.

Not sure about that Gerry. 1998 Coaster had an original radio and pa system. I replaced it with a 12v unit with no problems so I assume the original was 12v. There is something behind the dash on the passenger side (above the fuses and with a separate cover) that i assumed was a factory step down. Could be totally wrong.

I probably should have said that both the coasters I was referring to were HB30's; a 1988 and a 1992. I wouldn't have any idea about the later HB50's. In my defence I did think I should have added my disclaimer, but that it was in the middle of the night and I was worried about waking the "wind break". lol