I would go with the negative side of the circuit, as far more the most likely culprit, but some other possibilities include a distributor timing issue [if this is not a "computer emissions controlled" system], and a loose alt. belt* not charging the battery enough, (but charging enough to make the dash light go out) and if it is an LPS version truck, I would check voltage drops between the LPS disconnect switch and Alt output and battery posts.
* the alt. belt may also not pull strongly enough on the alt. to handle a loaded alt if the water pump is starting to fail at the water-pump bearing.