help with leak from top of windshield.

I got the same problem like you except mine leaks on the passengers side of the car. But it's in the exact same place just opposite sides. It mostly happens on heavy rain fall and when I power wash the car.

The cause of the leak is due to chassis flex of your car. That's why on most GT's you will actually notice a stress line across the A-Pillars where the leaks is.
The stress causes the glass to flex or move around in its' seating and this causes the sealant to flex and crack creating little pockets where water can get through.

Replacing the glass won't make the problem go away and is unecessary to change unless it pocked up from road stones or is cracked and needs to be replaced anyways. You'll need to take the weatherstripping off and the corner needs to be cleaned up and resealed using Black Sylicon Sealer you can buy at any automart for $8 a tube. You'll need a caulking gun also to put the tube in and press-out the sylicon. The gun goes for $7 and would be located on the shelf right next to the sylicon sealer.

I don't know how to take the weatherstripping off carefully. I understand you need a special tool so as not to twist/bend/ or otherwise damage the weatherstripping to the point you'll need to replace that too and that stuffs expensive. I think each piece may cost an average of $50!!!!

If you can get the top and two sides off you might as well re-seal the whole glass frame. But the hardest part is getting that weatherstripping off.

I'm thinking about leaving mine to a glass professional next year.

Do you have subframe connectors on you car? This would stiffen the car up and help prevent this problem from creeping back in the future.

If you do go this route and do it yourself, share with us how you did it. Especially how you got the weatherstripping off and then back on!

Good luck.