Check your voltage from the ground where you are reading 7.5 volts to the actual ground (truck chassis). Sounds like a floating grounding. It should be in your connector. If you have 12 volt on the hot then your probably have 4.5 volts on the negative which means you have no negative to ground but is back fed through the ground to another part of the ground circuit which is usually lights but not with LEDS so perhaps the backup device or license plate bulb. Bottom line: no solid ground to the vehicle.