I don't have a 756 Pro in front of me but I'm pretty sure the antenna connectors are mounted in the metal chassis and are at the same ground potential. I've never seen a modern radio that was not like this.
The IC-756 is a metal chassis radio and all connection points are at the same ground reference. Your
ground wire should be connected the the ground lug on the rear of the radio. (Stud with wing nut)
The radio is powered with a external 12volt power supply.
Problem with the internal SWR meter or a outboard meter ?