Like CyberSlug I can also confirm that for the older 4x4 transponder it works and beeps when it is up against the windhshield in the area next to the rear view mirror. I have ordered the new smaller transponder to actually mount it to the glass.

I can also confirm your confirmation of the CybrSlug observation (from mine).

I just retested the 'small' stick in the dotted rearview mirror area on the 241 toll road instead of the 91. Same. No beep. Defective stick? Heated windshield? Transponder reader not on? I don't know. But I do see entries on the FasTrak billing site.
So at least I know I'm not getting a ticket.

Maybe put the transponder in the frunk?

On the website it should show if it was a plate scan or transponder. That would let you know if the transponder is working but not signaling for some reason (unlikely, but worth checking).

IIRC, the rules for FasTrak say the transponder must be visible. I've never had an issue with it for several years of use, so it's not enforced much (knock on wood).

I just saw this post and am also on the east coast and use an EZ Pass transponder, which is similar to the SunPass.
I do not have a heated windshield, but just a regular one. Do you have any recommendations as to where to place an internal windshield Toll (EZ pass) transmitter? They normally recommend it to be near the rear view mirror -- just below or an inch on the side, from the stem of the rear view mirror. I would appreciate any one who has successfully used this to please shed some light.
I read another post which mentions the dotted area next to the rear view mirror -- is this the dotted area all along the top of the windshield?

See attached screenshot from the owner's manual. The recommended area is only the small dotted rectangles on both sides of the rear-view mirror. I also use the EZPass transponder in DC/MD/VA and have placed it in that dotted area on the driver's side, but I gotta say my results are mixed at best. There are a few toll-booths where the tag never registers no matter how slowly I drive through it. My vehicle also severely restricts the GPS signal. I am only able to use Waze/Google Maps reliably if I place my phone (Samsung Galaxy S9+) in a very specific spot (passenger's side cup-holder with the screen facing the side window and angled up). I never have any issues with voice calls but Pandora/Amazon Music are choppy at certain times leading me to believe that the data coverage is also being affected. Having said all that, there are a lot of people on the forums who have experienced no GPS/cell-reception issues, so it could just be my phone as well.

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Thanks very much - that picture helps. Do you have a heated windshield? I do not have one, and my cell reception has no issues with an iPhone 7, but I only use it for voice calls.
I will go to the main EZPass store in Raleigh, NC and I understand they have instruments which determines the best placement. If still a problem, I may have to get an external transponder. Will keep the forum updated.

I do have a heated windshield which may account for some of my signal reception woes Good luck with your EZPass transponder.