GM-GMC: electrical, gm diesels, gmc sierra

Question
I have a 1990 GMC Sierra with some electrical issues. The altenator was just recently replaced as well as the battery. Both are tested and fine. It seems that I am having some voltage issues beyond the battery. When driving, the guage never reaches 14, yet when tested from the altenator and across the battery posts- it is 14.3 to 14.6 continuously. When any draw is made (DRL's, heater, signal lights) my guages begin to act erratically and the lights all dim and the guage for the charge drops to well below 9 volts. A stereo system was installed after the fact, and it really seems to accentuate the issue. Help! Is it a faulty ground? The battery is not draining, and the vehicle still drives fine, but the gauges are making me nervous.

Answer
I know that 1990s GM diesels have grounding issues.  I do not know about gasoline-powered vehicles of the same years, but it is likely that similar problems exist.

You should remove the cables and clean all connections, positive and negative, and the parts that the cables attach to.  There are also ground cables from the transmission to the frame/engine and these should be cleaned, too.

There is also at least one ground cable in the dashboard, but I do not know where it is since I do not own anything that new.

The stereo installation is also suspect, since it made the problem worse.