Hello Michael,
The set up for my BJ8 is the black overdrive wire runs along the left top/side through one P clip. The P clip is held in place by one of the left center transmission tunnel top plate bolts.
Dissclaimer to being concours. This is how the PO had done mine.
Hopefully other BN1 owners will tell you how your model is wired. (Maybe with a P clip).
Happy Healey motoring.
Roger
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When I turn my Healey on, she turns me on.