I live in RI and have not put a front plate on yet, but I am sure it is a matter of time before some rookie pulls me over. Most cops check the car out but have never stopped me. Plate on the dashboard does not cut it here. I may fabricate a bracket that mounts under the bumper so I do not have to drill the bumper. I hear the first offense is around $85.
I live in MA also. Beware that they passed a new law regarding licence plate frames (Back only from what I've heard) that your not allowed to have them. Heard from my parents when they went for their inspection sticker. I guess if the framing around it covers to much of the plate they need to fail you. (Well the mechanic said it was retarded to fail for that so he was just removing them)
As far as the front plate goes i went to advance auto parts and found a nice fairly solid metal frame with "Ford" logo's in bottom corners and just screwed it in (Cried as i did it) I couldn't find any kind of actual mount at any of the local stores and didn't want to hunt to mail order one. Cost was 19.99 if i remember correct. If i was smart at the time ,now that I think of it, You could find a piece of foam or something to put between to keep it from rubbing against the paint.
I got the car in CT and the salesman (one that does actual paperwork) was a fellow mustang owner and I asked him how he got away with no plate. He just said he left it in trunk and tells them he just hasn't gotten around to putting it on yet. he also said they "usually" don't give him a hard time. So you can always try and do it that way but in time you will get pulled over for it.
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07 Mustang GT Metallic Alloy. + other spiffy stuff.