The return line to the trans is the horizontal line that goes back in. You can disconnect these without creating leaks. What year is your GP? If it's 99+ you'll have a quick disconnect fitting. Just pull the clip out with a pick and slide the line out. Replace the clip in the fitting and the line will snap back in to place when you put it back. What I did, and it may work for you if your fluid it noticeably different color that new, is I started the car and as fluid pumped out I poured new fluid in the dipstick tube until I saw fresh fluid come out. Then I dropped the pan and replaced the filter.
