Electrical Problem (97 Rodeo)
 

Alternator replaced and tested (off the car) and works perfect, battery is brand new.

Gauge in the car slowly moves down as I drive.

Hey I have the same exact problem, did you figure out what was wrong?

nope, went through entire car with a test-light, can't find anything. Got more work to do.

There might be a heavy duty fuse at the other end of your alternator cable that is blown, OR your alternator cable may be damaged or ripped apart. Disconnect the alternator cable from the battery or from wherever it leads to before it gets to the battery, and test to see that the alternator cable is putting out around 13 volts with the engine running. Sounds like a wiring or fuse issue to me though. If you don't find anything wrong there and the alternator isn't a total pain to remove, I would take the alternator off again and take it somewhere else and have it tested on another machine just to be sure.