When I removed my old converter in the '77 Safari, I notice it had three wires. On close inspection, two were 12v
and one was ground. The two 12v
wires differed in that one was highly filtered (good quality) but lower current, the other was crappy DC but higher current. Presumably the good quality circuit was for the radio/8-track player.
In some converters the 12v
output is divided, one leg for battery charging, the other for trailer circuits.
Either way, to hook up your two wire converter, you need to figure out which of those three wires are for 12v and hook both of them together to the 12v output of the converter. Just be sure they are both for 12v. One of them may be the charge line from the tow vehicle, which is OK.