I've had my switchable pass in the trunk the last couple of days as I was trying to figure out a good place to install it still. Logged into my account today and it was read properly every time I went over the bridge (4 trips so far). I have a brand new Model S, so I don't know if they changed the back windshield or not, but it definitely seems to be working. Even buried in my bag in the trunk, the system had no problem reading my tag (and since I don't have a license plate yet, that's the only way it would pick up my account).
That isn't a convenient place to switch the pass if you wanted to use the HOT lanes as car pool, but at least it saves the extra charge they tack on for "pay by plate".