I think this part does what you need:
http://vibroplex.com/contents/en-us/p243.html It's an 8 pin DIN cable with bare ends for the G-5500/5400 rotator control box. If I recall correctly, the ST-2 uses a 8-pin screw terminal block (yes:
https://www.foxdelta.com/products/st2-0417.htm). So the tricky part (wiring up the DIN connector) is done for you. A volt-ohm meter will answer which wire is which pin. On the ERC interface, the end user wires up a DB9 connector. (Who can still manage to get the pin numbering backwards. Learned the hard way! With the ST-2, a screwdriver is easier to fix such boo-boos vs desoldering a DB9.)
Hope this helps.