GM-GMC: Alternator, negative battery cable, fuse block

Question
ok i have a 1988 GMC subarban 1500 5.7 350. I brought two rebuilt alternators, put them on and a new battery and the alternators still drain out the battery. I also check the wire from the battery to the alternator. it read 12.58 volts. what else could be the problem.  

Answer
There has to be something connected to the electrical system that is staying on and draining the battery.

In the old days, it was a dome light or a glove box light left on.  New vehicles have more electronics and more places for drains to occur.

If you have access to an ohmmeter, set in on the 2 Amp scale and then disconnect the negative battery cable and install the leads from the meter between the negative battery post and the cable.  You may have to select a lower amperage scale, but you should get a reading.

Then, disconnect the fuses in the fuse block, one at a time, until you find the circuit that cause the power draw.  That is where the drain is that is killing your battery.  Finding the exact source may be harder to do, but at least you will know where to start looking.