I've been battling this for quite sometime now and I go through the 110 FWY everyday. Mine is located by the right portion of the rear view mirror but it does not work all the time, thus, incurring costs as a single HOV vehicle everytime I pass by (not cheap). Going northbound in the morning is around $10 one way.
One day, I got curious. I went to my metroexpresslanes.net account and saw massive charges on my account. I called the customer service and explain the situation. Apparently, they knew about the tesla problem. Below is what they did:
1.] Reversed all charges associated to my tesla license plate.
2.] Apparently, there is an exclusion list on their system and requested my license plate to be added to that list. The purpose of this is obvious, to exclude your tesla from being charged all the time. This does not take into effect right away. you need to follow-up on a weekly basis.
Hope this helps.