You mean like this one? This is a 66 Plymouth with a "Foot Patrol" sign on the roof. The car is assigned to the TFP, Tactical Foot Patrol, that would be assigned to areas in need of additional foot soldiers.
The car would be left on a block with the light activated, thereby citizens would know a cop or two was in the neighborhood.
A similar set up was used by Baltimore and Washington Metro PDs. The BPD Chevy shows the light a bit clearer.
Another unusual roof set up that was used by many departments in the 60s was the roof recall light. Shown below on a Baltimore 66 Ford, this light would be activated by headquarters via a tone encoder that would let the officer know that HQ was looking for him when out of the car. This was before the wide use of portable radios, and was only efficient if the officer was close enough to see his car.