How do I get the black trim peice off the outside to get to the window to find the leak?
98 Ext Cab w/ rear sliding window.
Already checked around the cargo light, no sign of leaking.
Dripping down from the top of the window just to the passenger side of the rear slider.

 

I am having the same problem, and have no clue how to get the rear window out either!

 

You need to pull the interior around the back window. The black trim stays in place. If you have factory original sliders, you'll need a new window/slider assembly. The factory originals crack the frame and can't be sealed up. You also need a roll of 8mm (5/16") butil sealer like 3m window weld. It's a common issue, but it's been a long time since I've read much about it here. More info is available if you search "leaking slider".

 

So you need to remove the window from the inside. Hmmmm ok I will look at this.
Thanks

 

Just got an estimate from a local windshield replacement shop and they quoted $300 for a "new" replacement slider installed.
The passenger seat on my 99' scab comes out then the nuts/fasteners come out from the inside of cab. They showed me a new slider window and said they've done "many" replacements.
The new ones will not crack or leak, so they say and do give a warranty with the new install.

 

Just did the girlfriends truck a few months ago the shop we took it to did a repair on it, used a in house adhesive/rubber based compound and sealed it up like new, didn't cost that much either $150 or so... new frame installed was going to be 3-400 with all new seals and sh*t if my memory is right (probably not, lol). But he also said that it was a Ford problem, lol, very common but once fixed he says it shouldn't happen again.