Just to make sure it's a 70 AM, count the number of contacts on the harness connector, there's (5) small connectors. (Don't want to insult your intelligence, but the 70 am's are often times confused with the 69's which are different.) It should also have a green band painted at the top of the lens, NOT blue.
Your best bet is trying to get a harness from a junker. I worked at an radio install/repair shop and no current manufacturer makes the harnesses. You could try the vintage car wiring supply/harness companies like M&H fabricators, but I doubt they will have anything.
Another thing to remember is there was different harnesses for AM's. Is your car a one speaker or two speaker set-up?
Joe
-auto radio repair tech
-Creator of Team Chevelle's radio info pages coming soon...