Pretty sure that plug goes to the neutral safety switch which is attached to the shift lever mounting bracket. Also controls the backup lights. ground wire goes to floor sheetmetal.
Neutral safety switch was never on the shifter. It was always on the transmission (automatic - won't get into the clutch safety switch). In late 1968 or 1969 beginning of the model year, the backup light switch was moved from the shifter to the transmission and shared a single part with the NSS.
