I don't know what fuses to check, meaning, I've checked the ones in the glove compartment, and trunk. None seems to relate to this.

When you turn the key to position 2 before starting, does everything seem to work? Message display, temp and fuel gauge?

Yes, it does. Battery light is always on, no flickering or anything. What pisses me off the most is that I told the shop owner to fix it and he told me they can't since they don't find anything wrong with it. The only "solution" he gave me was to change the alternator for a new one, of course, they'll do the work and charge for all that.

I didn't do the timing because I don't have the tools and time, but, certainly, I'd rather do it myself if I could go back in time.

You mentioned that you rebuilt the alternator with Bosch parts. Which specific parts did you replace? Carbon brushes? Slip ring? Voltage regulator? Bearings? Stator? Other? Why did you rebuild the alternator in the first place - was there a specific failure or problem that you were working to resolve?

I changed everything, and I did it bc it was already out while changing most parts of the cooling system including radiator, also I was cleaning deeply every part were coolant goes in (last owner had regular water in it only and there were oxidation all around). My alternator is the water cooled and is a pita if I needed to do it later, so I decided to did it as preventive maintenance since I had all the parts. Remember my car has 16 years already and I've found that a lot of parts are still the ones that came from factory. I change things before they breake.

So guys, this is what I've tested and found so far... after doing some more research, I tried a load test to see voltage with warm engine (over 80° C). Before the load test, at idle, it was steady at 13.6 volts. Then, I started turning everything on, lights (I've xenon), AC, radio, emergency lights, foglights front and rear, wipers, defroster (rear) and interior lights: voltage dropped to 11.9 volts jumping to 12.6... This to me is a signal of a failing alternator, right? I'm mad because the alternator was good before the shop job, and now it isn't.

Also, she started to behave while accelerating, like hessitation or sputtering, and when I scanned her today, it now shows in ISTA-D error 151, Fuel Injector, Cylinder 5. I don't know if the injector is failing or is just clogged. Damn!

Sounds like a failing alternator. Low voltage can cause your other problems as well. These trucks don't like low voltage.

Well, after fighting with some silicone residue used to seal the alternator and its cap as well, I finished changing voltage regulator again with an oem (Bosch), and diode pack. Brushes seemed ok as well aaaand.... The light is still on!

So, it is not the alternator as I suspected at the beginning.

I know the light comes on when voltage from terminal 30 and 15 are not the same, but, I dont' know what else to try, voltage on battery posts is the same as in the alternator bracket.

I've no clue about this. Is so frustrating.

To properly test an alternator you need to load it and measure amperage. Measuring voltages when not taking current into account is going to throw you off, as will not taking into account the ground side of the circuits. Maybe check wiring diagram to see if alternator or DME is in control of alt dash light.

Ok, I think I found the issue, it is not the alternator, v belt, or grounds. Terminal 15 is reporting about .5 Volts less than what the battery has.

Now I'm chasing what in the middle can cause the voltage drop. Trying to find a diagram to check further. At least I now have a clue.