1990 Ford Mustang selenoid clicks

My selenoid on my 1990 mustang when turned over clicks but my starter itself wont crank. My cars alternater has a slow drain ive noticed. So we let the jump cables run for 10 minutes between start attempts. But it wont turn over. When I take the cables off it only has enough juice for maybe one more attempt. Ive asked around but no one has given me a clear cut answer. Ive got a new starter, but I havent put it in yet. Put in a new selenoid even though I knew that wasnt the problem. Connections and cables are fine. Any ideas?