Oldsmobile/Buick Repair: 1999 Ford F-150 4.2L XLTsupercab- electrical, ford f 150, amp fuse

Question
HELP!!! I hooked up a 1200 watt amp and 2 subs to my truck(have done this many times before on other vehicles). The same day, on my way home i noticed that the battery light came on.  First, i replaced the battery(high output in now). Second, i replaced the alternator (a diode tested bad). BATTERY LIGHT IS STILL ON! Also i noticed that when i DO get the truck started all the needles on the guages go all the way up for a sec. then they go back down, then normal?!? I also noticed that if i move the under-hood fusebox to a certain spot the alt. will start to charge. What is wrong with my '150? Also when i first got the truck, the battery cable going to the starter was so corrided that it fell off. So a buddy of mine wired it direct w/ a fusible link. (starter is always getting power!?) I know that is not the correct way, but it needed to be fixed asap and that was the best we could do for it at the time.  Plus he said that it wouldn't hurt it a bit.  It hasn't seem to change anything, maybe it has nothing to do with the recent "battery light" issue. Just wanted you to know everything that has been repaired concerning any electrical.

Answer
Hello Aaron,
It might make a difference as to where the fuse link is, and the amp fuse in relationship to it.
I am familiar with fusible links, but having one in the starter cable may be a little light, as compared with a good battery cable.
And if the alternator output needs to travel through the fusible link to get to the battery, you may not be getting all the goody out of the alternator.
I'm sure your buddy went to a solenoid, or the starter would run all the time.
Van