Just walked by the M.A.R. and the lights were on at 10:00 p.m. They had just closed so I had to resort to a brief read of the new supplemental menu in the window. The menu features a good list of appetizers, a section of enchiladas and tostadas and a good sized list of entrees in the $7-9 range, including at least one mole. No need to worry about the authenticity of the tacos because they seem to be authentically not on the menu. Sorry I don't have more info, but what I can say is that to me it looks highly promising.
The M.A.R is between Central and Harvard in Cambridge (closer to Central,) near the Plough. It had been basically an appealing but unspectacular diner, offering breakfast and lunch and closing at 6:00. It's now open until 10:00. Can't say if both menus are available at all times.
(apologies if this has been posted about already)