Ron Mahoy should be able to help you. Indiana State Patrol used them in their Mustangs. IIRC, I found an NOS one for sale online a few years back and sent him the link.
He doesn't really post here, but you can PM him at ORG (ChipsMahoy).
Here's a thread on this site with a pic for reference:
http://www.specialservicemustang.net...ad.php?p=12114
Also, could have been an early overhead camera system (slim possibility).