FasTrak device is an RF ID module for paying toll bridges and express lanes in California. Some people are asking, how to mount it in MX-5.  It is recommended to keep it on the wind shield, which is already cramped with Forward Sensing Camera and Rain Sensor. The FasTrak devise is quite big and ugly in my opinion, and it is not necessary to keep it on the wind shield.

Soft top MX-5 is RF transparent, so you can keep RF tag inside the plastic cubby and it will work perfectly.