I mounted the E-Z pass on the front windshield, went through 3 different tolls and none were able to scan. Is it just me, perhaps where I placed the pass, or the 991's windshield actually causes the interference (antenna, etc.)?

 

I haven't had any problems with my ezpass mounted to the windshield (behind and to the passenger side of the rear view mirror).

Where do you keep yours?

 

Lower driver's side, right behind the registration sticker. I always put it there on my previous cars - never had any issue - figure the sticker blocks the EZPass and makes it look cleaner - really don't like the big ugly white thing in the top middle of the windshield. Unfortunately I don't plan to mount the front plate, so the external front plate sensor/tag is out of the question.

I guess the 991 is too low?! Especially when I put the pass at the lower corner?!

 

here in ca, u dont need it. the camera picks up your plate and charges to your account

it's EZ pass here too

 

yeah, half of the toll plazas here do that if they failed to read off your pass, they just capture your plate and charge your account.
However, there are plazas with the stupid raise-bar, it needs to read your pass before it let you through - stupid and causes unnecessory traffic. I hate to be the guy sitting in my porsche and waiting for the transit cop to walk over (so f'ing slowly) and scan my pass physically... while a million of cars behind me honking... and yelling "Hey ahole, move it!!!!" (and probably thinking DB in a Porsche, how typical)

 

You have to mount it higher on the windshield. They make a black box/holder device the you can slip it into on your windshield.
Don't do what I did, paint it black, worked for 5 yrs, then they confiscated it one day, freaking hassle.