Coming back from a weekend trip heading down a canyon I noticed all my lights were getting dim.

I immediately turned off power consuming devices, fan, stereo, etc.

Luckily I have a manual and would coast on the downhill areas with the car off. On the flat areas, turn
the key, put it in gear and pop the clutch and I was off until my fuel injection shut off.

Well I limped down the canyon and made it out.

Back home I took the alternator out and went to get it tested, passed with flying colors. Took it to Autozone and they passed it too.

Put the alternator back in and took the car down to have it and the battery checked with the car on,
it passed again.

Any ideas? I thought a bad alternator was an all or none situation?

Could it be something else, a relay?

As far as wiring, I've replaced the engine harness and put in a positive battery cable, but I did these things a while ago.