I have an '01 King Ranch, and lately whenever we get any decently hard rain, (which happens quite a bit in Ohio) my back window leaks. Now I was wondering if any one else has had this problem, and what you would recommend. I thought about just taking some silicone sealant or something of that nature. Any advice would be greatly appreciated!

 

Sounds like your 3rd brake light! Check the seal around it and more than likely thats your problem

 

I too have a '01 KR with the same problem. I just got the rear window resealed by Safelite Autoglass. It was only $80.00. They had to pull the whole window out and reseal it with a 3M brand "kit". Apparently 1999-2003 f150's are known to have this problem. The technician said Ford used a foam type seal that doesn't work as well as the 3M kit (which is a roll of 5/16" wide by 15' mastic looking stuff.) You could probably do it yourself, but you would run the risk of breaking the glass and there is no guarantee to be leak free if you do it yourself. If you are lucky, it could by the brake light. Mine leaked on top of the passenger side though. Here is the link to the stuff Safelite used if you are interested.
http://www.amazon.com/3M-8611-Windo-Weld-Ribbon-Sealer/dp/B000CQ4AOE/ref=sr_1_1?ie=UTF8&m=A3TW1WCPSO49LP&s=automotive&q id=1270350575&sr=1-1[/URL]

 

Get someone to hold a water hose above your brake light for at least 5-10 min all around the light. If nothing happens, then run the hose around the backglass seal. Ive replaced these and the leak can be on either one, but normally its the backglass.