I recently needed to check/adjust timing on the same model you have, and had the same question since the service manual (Haynes) gave a location description completely at odds with what I was encountering. A Chevy expert said it was a tan wire with a black stripe located under the dash on the passenger side close to the heater and next to but separate from a larger wire bundle. He was spot on. With the connector unplugged, and engine idling, set the distributor for zero degrees. After locking distributor and a final check, reconnected timing wire and was good to go.