Aftermarket radio's have proprietary harnesses that plug into the back of then. The old days we would hack up the factory harness and use twist caps to connect the appropriate wires. The past 10-20 years the trend is to use a special pigtail harness to bridge that gap between the factory harness and the proprietary harness from the back of a modern receiver.
Search eBay or Amazon for Scosche GM01B, it's a harness for 1978 -1987 General Motors Radios. It will come with two pigtails, the smaller one does not apply, it taps into a separate factory harness that provides power for the clock, which 78-79 was not wired for. I'm going to guess that this may be what you got from Ames as well, if you have the Ames part number that tell us what harness it is.
You also need the proprietary pigtail harness that plugs into your aftermarket receiver. If you purchased your receiver new, it will be in the box. It should have instructions what wire goes to what speaker, power, etc...
I still use twist caps for these application specifically, but more and more I see folks either solder or use butt connecters, which would be an improvement.
I will be wiring up mine in a couple weeks and can take some pictures if that help.