There are other threads on leaks, but they all say to do what you have already done. What I did to see if it would stop was put black electrical tape in the top back corner of the door jam, where it meets the roof and the corner piece. It seemed like the water was running down from there and dripping off the plastic onto my rear speaker. After I put electrical tape there, I waited for the next big rainfall, and ta-da, no more wet interior. It's worth a try.
I also have a 1998 Ranger Regular Cab.
2008 F-150 Supercrew Lariat 4x4, 5.4L.
2007 Ranger Supercab FX4, 4.0L, Accident mod done in December, 2012, totalled.
1998 Ranger Reg. Cab, 3.Slow. Sold, then totalled.