It should be a black wire that comes off the three-prong plug on the back of the bulb, and screws to the body nearby. I have a 78 Spider in my driveway I can look at later today. But basically take the light out and follow where the wire goes. It shouldn't be far.
From the wheelwell end, there's a small accesss opening. I'd try the front first by removing the light, and perhaps the bucket, just three screws.