1988 ford Mustang 5.0 ltr lx clicks rapidly?

After the summer I parked the car went out to start it after a very cold week battery was dead I trickle charged the battery then the car started clicking rapidly we you try to turn it over I have tried to jump it to no avail mustang ran fine when it was parked

Pull both of the cable clamps off the battery posts & scrape both the inside of the clamps AND the sides of the posts. If that helps, most all car parts stores carry a small wire brush designed just for that job.