I drive the southern half of Route 128 almost every day during commuting hours. There are many unmarked cars of various makes and models from chargers to Mustangs to SUV's (not just Crown Vics - but there are plenty of them also)
At the very southern end, they sit in a crossover about a mile north of the intersection with Route 3 (Braintree triangle).
The next typical northbound spot in on the grass at the Route 28 on/exit ramp.
And the next typical northbound spot is underneath the Route 95 overpass ramp and sometimes in the grass at the University Ave on/off ramps. (Right now they are there all the time because of construction work but don't seem to have their guns on.
Further up they can be found northbound around the Route 1 entrance/exit ramps and then again at the top of the rise just before Route 109 and then again go down the hill just after the 109 ramp complex.
The next spots are on either side of the Needham flats from 109 to Highland Ave and also at the Highland Ave on/off ramp complex.