Swapping factory AM for factory AM/FM

you could always not use the factory wiring.

The FM radios used door speakers as they had left and right channels. You can get the "speaker crossover loom" for a 65-70 and it should connect directly to your radio's speaker harness if it has the 3 pin flat connector (one male, two female at the radio). This is available in reproduction from several vendors. Then you will need to get the door wire harnesses out of a wrecking yard as they're not available in reproduction for the 71-73 cars. If you're junkyard shopping, look for a Cougar as most came with door speakers. Otherwise you can check eBay. Another option to consider is to get both a radio and speaker crossover harness from a mid 70s to early 80s car that has both door and rear speakers. It may take a little time to unweave it from the underdash area and trunk, but you can get a factory AM/FM and possibly AM/FM/8 track for cheap that will plug into the factory power and light connector, fits like it grew there, 4 speaker stereo sound, all without splicing one wire. Of course you still need to come up with the door speaker harnesses. That's what I did with my '69, using a 78 T-bird donor. It may not have the volume or clarity of a $500 modern head unit, but it looks factory cool and works great, all for under $100.