We had a similar problem on our 1992 RR: The tach would intermittently work, and especially at stop lights or when the car was in drive but stopped (Like at a light) When it stopped working, you would hear belt squeal. Giving the car gas (even with the car stopped in drive, foot on the brake) would cause the tach to work again, and the noise goes away. The tach is in fact driven by a signal from the alternator, one wire, correctly described in a previous post. On our RR, sometimes the squeal would become very bad and not go away even when driving away from a stoplight for some time. You also could notice a burnt rubber smell occasionally. Turning off electrical accessories usually alleviated the problem. A failing tach typically means a failing alternator. We had ours rebuilt for $50. Problem came back and it was one of the brushes. We put one of those Goodyear "Gatorback" quiet belts on with the sideways grooves (local auto stores) and tightened the tensioner, but not overtightened it. We also put a Wal-Mart battery in it (The yellow one (Maxx?) like 1,000 CA, the only Wal-Mart batt that isn't junk.) This solved the problem. The tensioning device is extremely annoying, and has to be just right. The book says like 4mm of belt deflection when pressing in the center of the belt. Since the a/c in the truck doesn't work, we also disconnected that belt. At night, do you ever see the alt. light come on, just very dimly glowing red? That's what ours did, never on bright, just very dim and it got a little brighter like when you hit the brakes which requires a lot of electricity.
The other thing you can do to temporarily help the problem is adjust the idle throttle up to like 1,000 RPM so it might not squeal as much.
Remember--the alt. belt CANNOT be TOO TIGHT! This will smoke your belt off just as much as a loose belt can.