Unfortunately, '91 is entirely different than the '77.
It's easy. Use a voltmeter, find the accessory terminals along the top of the fuse panel, use one that is 12V constant for the radio memory wire, and use one that is on in every position but off/start for the main power. You can use a spade connector to fit in the terminals, or you can go to the wrecking yard and get stock connectors to lock in place, which can also allow you to "piggyback" connectors if there are no empty slots.