What to do if I put 1 quart of the wrong transmission fluid in a 1999 ford f-150? Do I need to flush it or can I put the rest correct mercon 5 in

1 Quart of the wrong type in there isn't really going to bother it. Automatic transmissions have many quarts of fluid in them. You'll be fine. I wouldn't drain it out for one odd quart. Just make sure you put the right fluid in the next time. You'll be fine.