Maybe you have a deck-mount converter . FWIW, my deck mount converter was in a rear storage bin. I have heard some have them under a bed, You can look at the 12V distribution panel with the cover off and see which 12V main cables go where. If you have a single pair of 12V cables and no termination block, wires traveling towards the battery location, you may not have a converter. If you see two pairs of larger 12V wires attached to lugs together, you most likely have a converter.
I would guess you have a converter. For mine, it was an option but that is in 1971.