I think the reason Ford designed it the way they did was to have a circuit protected system in case of shorts. With the ground system, the wiring has no protection. The other situation is that the top would be just like the starter in the respect that it could be operated without the ignition switch on. Any shorts in the system and the power top motor would start running. Just a lot of "maybe" situations but the Ford folks had their reasons I guess. Some of those pumps are mounted back in the trunk area so the wire harness would be longer. In the case of the Lincoln & Mercury cars of the 49 thru 51 era, the pump was mounted in the lower left side fender well area. They had hydraulic seats and windows as options in those cars as well as a power top on convertibles.