I purchased a Pro-89 off eBay and downloaded the software to program the
scanner with my PC. However, I do not have the scanner-PC cable -- as I
recall, the cable has a DB-9 serial connector on one end and a two-connector
3.5mm plug on the other.

Does anyone have one that you would sell, or, can anyone point me to a
schematic so I can build my own?

I'm trying to find a schematic through Google without much luck. Thanks.