autiger4

I have a 2005 v6 mustang that has randomly not been cranking for the past two years. Most of the time it will not crank on cold mornings (not even try to turn over but radio and lights will come on). By mid morning the car will usually crank and its not every cold morning. Over the past two years the car has probably spent at least a month in the ford dealership and they say they can not figure out the problem. Last January I took it to a private mechanic and he figured out it was the fuse box and thought that the four prongs on the starter fuse wasn't fitting correctly so he told me to take it out and put it back in and it should start. That has worked most of the time except two when I couldnt get it to start. I was going to have it towed yesterday and the tow man jumped it off by connecting the negative cable to the chasis and the positive to where the fuse box cord connects to the box. It seems my fuse box is not grounded. I am no car expert and was hoping someone could give me some advice. thanks