You own a 1600, why should I talk to you??
Just kidding. Call a local place that installs sunroofs, our auto body repair. I know that they insatll seals, it may be a pretty standard thing that they can install for you. On the other hand call your local Nissan Dealer and ask. And if you have the bux, have them do the work too.
Where is it leaking? I had this a while back in winter from half way up the a pillar and the drips would run down and fall in my lap.. s##t. Came from the T-section at the top of the window glass. WhaT I did was clean the rubber, trim any little burrs of it and then silicone protect it . This seemed to do the trick. Someone also suggested raising the window glass by so that it applied a greater pressure to the rubber.. ( think it is adjustable along the bottom edge of the glass) I didn't need to do this but it could save you the $$$