The Alternator is an Errand Item in Postal 2. On Monday, when the Postal Dude gets ready to complete the errands of that day, he realizes his car is not working. However, he does not do anything to get it repaired until Friday. He heads to the Junkyard in order to get it. Once there, he has the choice of paying $500 for it, or stealing it (which causes Mike the owner to release the dogs). Either way, a water tower collapses and blocks the exit, and more dogs escape from the pen, causing Postal Dude to find a different way out.
The alternator indeed worked, since in the end of Apocalypse Weekend, the Postal Dude is seen driving his car along with Champ and his trailer, escaping from the nuclear explosion that destroyed Paradise.