Harry's Jeep

Harry's jeep has been out of commission for over a year and a half and we just couldn't find the problem. This past weekend we played around with it and put in my battery from my jeep and his jeep started right up. We previously installed a new fuel filter but didn't think that was the problem.. well with my battery the jeep started up great and ran perfectly. I have to back track a bit.. problems with his jeep started when we installed a radio. The battery was dying and we didn't know why. We sent the jeep to the jeep dealer for them to look at it and after three weeks they couldn't find anything wrong.. they said the battery was bad. It was a new battery only a few months old so we took it back to Auto Zone where they did tests and said it wasn't bad. Well we went back and forth for months trying to figure out what the problem was..but nothing made sense.. we figured it was a battery but they kept saying the battery was fine, then we figured it was the fuel system and replaced the fuel filter but still nothing.. anyways we finally replaced the battery with mine and the jeep started up with no issues. So I took the battery back to Auto Zone again for the fifth time and had to run a test. After work I stopped by to pick it up and they told me that the battery was bad and would replace it since we are still in our two-year warranty. So after all that the battery was bad, which we said from the beginning. Now we have to install the new battery and get his jeep smogged before he can drive it again. Oh and while we were checking out his jeep we noticed his transmission is leaking.. if it's not one thing it's another...