PLEASE DO NOT throw parts at a problem. Having places like XXXXXX..one test components is a surfire way to invest a lot of money.

As Andrewh said, a good charged battery --engine off--should be around 12.6V. A properly charging system, measured directly at the battery posts, should be --at a good fast idle--and with all accessories off--around 13.5-14.5, depending on temperture, condition of the battery, etc.

Look through the technical section--there are a number of articles on the wiring problems that are now cropping up on these old rigs.