The problem here is that the parts stores are trying to read your codes with an OBD2 scanner because they are fooled by the 16-pin wiring harness. They probably have the right equipment, they just don't know how to use it.
You need an OBD-1 scantool with a 16-pin wiring harness.
This link provides free software to use on your car. Make sure you have loaded in the correct datastream for your car. The only part that should cost money is the cable. This guy even has a link so that you can buy the cable for your car.
I would be interested in trying it out myself - but I already have a scantool.