Charging system failed can't do anything

[criptich]

My 2003 Ford Expedition EB edition has some very strange problems.
I was driving one day, then a 'check charging system' warning came on in the digital readout of the dash board.
Roughly 5-7 minutes later the battery meter was dying(lowering) right before my eyes to empty(decharged)
i parked right before it ran out and could not start the car again. when i realized it was dying i had turned off all electronics but it didn't seem to help.
I got it towed back to my house, we replaced the battery AND alternator to a good alternator, had it tested. it didn't fix the problem then we discovered another problem.
With the new battery and alt in, the batter meter shows the battery is charged but the car wont start/turnover now until i use a battery charger in the garage. What i mean by this, the battery had low voltage until we pumped up the volts back past 12.1, could only get it started at this point.
i turned off the car and tried to start it again, voltage was back down to 10ish.
so after all this the problem lies with the charging system.
The replacements i've done so far are new battery cables, new alt and battery.
we checked the fusible link and its in good condition.
The thing i've narrowed it down to is some little 5 amp fuse ive heard about somewhere away from the fusebox that keeps the charging system from working. -i dont know where this fuse is though..

The things ive tried/trying to do are:
change alt to good tested alt, changed battery, changed cables, tested fusible link-came out good, checked ground, and about to change the plate which the cables rest on the battery of the positive side.
im TRYING to find this little 5 amp fuse though, if anyone can help me find that please.

[ASE AMD]

Check ur voltage output at the alternator. Also check for field voltage on small connector.