Sorry to chime in so late. I had mine done (had a tiny stone chip/crack upon delivery).

It was explained to me that the replacement windshield comes assembled with it's gasket in place. 

All they have to do is add a bead of sealant, connect the antenna and drop it in.

Might be that your guy is screwing up by re-using glass and adding the gasketing himself? Maybe not completely removing sealant? 

My point: the way things are set up, the replacement unit is complete so should simply drop in for foolproof results

Smiley

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2007 997 Turbo