If you're up for swapping out parts, I have a '90 ECU and AFM that I have sitting around collecting dust. They both worked when I pulled them from my car back in 2003, and have used them to troubleshoot a car here and there over the years. They both have ~35k miles on them. You're more than welcome to try them out for a few days if you get out here to pick them up.
Sounds like the O2 wire has a break in it somewhere though, whether in the casing causing a short, or the actual wires inside the casing are broken.