I just paid my auto excise tax online at the City of Cambridge.
The Payment Confirmation page include the following sentence:
If you are being held at the Registry of Motor Vehicles for non-payment, please contact the Finance Department at (617) 349-4220 to be released.
Also, they had a great web interface, you have to type in your auto registration number and it is case sensitive!
you have to type in your auto registration number and it is case sensitive
aNOTHER thing I didn't know about Massachusetts -- you have upper and lower case characters in your vehicle license plates? I'd comment that "everything's up to date in Kansas City", except that's the wrong state.
well done