If he is working on that 1992 f250, then he doesn't have a instrument voltage regulator. They stopped using that system starting in 1987, except the vans which was brought up in a recent post. 1986 down trucks use the instrument voltage regulator. That is also why the fuel sending units won't interchange from 86-dwn and 87-up trucks.
To the original poster; Has any engine work been done lately? There is a ground wire that goes from the back of the engine to the firewall. That is often forgotten and not re-attached if the engine has been swapped or the heads have been taken off. The cab needs a ground, that is usually one of the few spots that it gets a ground, since the body mounts are rubber.