FWIW, I had been using an external likcense plate FasTrak transponder here in the SF Bay Area, as I could never get the internal version to work. I had this mounted on the black front bumper bar behind the nose cone. Worked great.
There are soon going to be new HOV/flex $$ pricing lanes here, so there is a new internal FasTrak Flex transponder with a 3 position switch. 'Ordinary' drivers set the switch to the number of occupants and use the bridges/toll lanes, and are supposed to be billed as appropriate. But what about the shielded Model S windshield--what could I do?
I went over the Bay Area FasTrak web site and read up on the system, and also spoke with a representative. If you have the white HOV stickers and thus qualify, you can get the new FasTrak Flex transponder and leave it set to 3. So I got the new transponder, set it to 3, placed electrical tape over the switch and the speaker opening, and also taped up the enitre perimeter of the transponder case. I then popped my nose cone off, deinstalled the old license plate transponder, and Velcroed the new one to the inside of the nose cone itself. I have not yet hit a HOV/variable pricing lane yet. YMMV.