The MKIII doesn't have OBD-1, it's diagnostic system predates the California state specifications known as OBD-1 and is much more primitive... there is a serial out within the ECU, but it is not something available even to general service techs; it's pretty much just used for diagnosing the ECU itself.
Your problem is VERY common and is usually solved by replacing the knock sensor wiring. I'll find you a link to a writeup on the rewire and post back.
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