Well , you might try running a ground wire , from some metal part of the cab, like where something's bolted on under the hood, directly to the negative battery post.
Try the one for the CB. Our repair guy working on the fuse box had the same problem and ended up connecting his tester to the CB ground. If you're trying to ground an inverter, good luck. Haven't found a way to do that and have to use a 12-volt plugin style.
We Hire: Repeating what pawpaw said. I run ground straps in several places on my truck. Electrics get very whacky if they cant get to ground. A good ground is as important as the hot wire feeding. Running a couple of ground straps costs pennies but the benifits can be very important.
