Actually I think this is a very common problem on GTV's, the whole ventilation system on non air con cars isn't really that great.
Mine does this too, the rear screen joins in the fun also. The settings are not on re circ and all the fans heaters etc are working correctly, ie there are no faults or finger troubles.
The Front Screen de mists in a reasonable time once the engine is up to temp, perhaps a bit more sluggish than most cars but you can't have everything ! The rear screen and mirrors are pretty quick. To help things along I have tried " Fogex " and I'm not sure that it helps a great deal but how can you be sure ? If this stuff wasn't working maybe it would be worse ?
I also use one of those household dehumidifiers ( Silica Gel things ) and during the winter months I stick this on the parcel shelf, not the best place for it but you're not exactly spoiled for choice in a GTV, was unable to find a compact one which would fit in the door pockets which would be ideal. I think this does help, for what they cost it's worth a try.
I believe that this is just one of those foibles and there is no cure for it other than having air con. This is not an option by the way, if you don't have air con you can't do a retro fit.
Best cure of all ? De mist sufficiently to drive safely then go out for a thrash around and get everything nice and warm and dry and have lots of fun whilst you're doing it !