Actually, the 66 and older have no relay. They are live off the battery and their is a circuit breaker mounted on the firewall.
67 and up use a relay inside the car that makes them work off the key. I think you are correct that it's behind the kick panel. If nobody replies, I can check the assembly manual microfiche tonight. I think it will be on there.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars