I don't know the Pro-X specifically, but by the looks of it you're at the right spot, aftertouch should happen somewhere there - and this single cable should be sufficient to carry all signals (usually 2 or 3 pins/wires are used for this type of AT).
Some measurements would be in order if you have the service manual, or going into test mode (after backing up your stuff) might be revealing (it has a specific part for aftertouch). The flat cable might also have some rupture on the traces, it is easy to damage the thin conductive trace, so do a careful inspection - possibly some continuity checks.