My father-in-law has a mid-late 90's Ford F150, and here lately his truck won't start when it's pretty cold out (15f and colder). We have to jump him off. It's only when it's cold out that it does this. He just had a battery put in yesterday, and then it did it again this morning.
I will say it was his boss's friend/mechanic who put the battery in. My father-in-law said he jut brought it out of his house.. so who knows if it was new (it didn't looked used), or the right battery.. but the old one was doing this too and it was only about 3 years old.
What else could it be? Alternator perhaps?