Most states have laws about bumper height, tints on side windows, tires past the wheel wells, loud exhaust, loud stereos, but in general I've found that most cops will not bother you unless A) you are doing sumthing to rouse suspicion...being a dink peeling out, loud music, racing along the street, generally geeking off. B) there is something blaringly wrong with your vehicle C) they are very bored. Regrettably, being a cop can be very very boring at times. So, to keep themselves busy they "look" for potential issues. In a big city, most cops done have time to worry about "Are your tires sticking 3" too far out or 2" like the law allows" So, alot of city people get away with a lot more simply because the cops are busy. But once you go into the more rural areas things are different. Then you come to NJ and you can't get away with anything. Especially if your stopped by a Trooper. They are generally very courteous, but you get NO leeway with them. (in my experience)
I've had some very bad experiences (having my side window smashed, pulled out through the window, slammed to the ground, handcuffed, and manhandled very roughly...cuts, black eye, sprained wrist, bruised ribs... while surrounded by 4-5 cars at gun point) ...case of mistaken identity in case you guys r wondering... and I've had some great experiences (Cops and EMT rushing into my mothers house to help me get her out of bed onto a stretcher, down the stairs, then escort us to the hospital because she was having a heart attack) so it goes both ways.
What can you do to limit your risks for being dicked with? As others have said, stop the engine, keys on the dash, hands on the steering wheel, music off, all windows down (I dont care how cold it is), stay quiet unles spoken to, you tell the wife, girlfriend, chick, whatever before the cop gets to the window to shut the fiddlesticks up and not say a god damned word until its over or you will beat the poop out of her - just joking, but make sure they are quiet too - Do not smoke, do not drink soda, water, anything, if they ask to search the vehicle make sure you tell them up front if there is a gun in the cabin and exactly where it is and whether it is loaded or not. If it is in the trunk you dont "have" to mention it unless they ask to search that too. And for gods sake swallow the roach in your ashtray and dont let them find shakings on the floor dammit, thats just wasteful. Roll it in the house where your supposed to!
Finally, In NJ all fire arms must be LOCKED (as in with key or combination lock) in a CASE (soft or hard doesn't matter... felt bag = no) in the tunk or rear of the vehicle if the vehicle doesn have a trunk (Jeep Wrangler). Ammunition must not occupy the same space as the firearm unless it is also LOCKED in a SEPERATE case. Firearms cannot be loaded under any circumstances. Firearms can only be transported from your place of residence to the place you intend to use them (target range, hunting grounds, etc) You can actually get a ticket if your found say with your handgun in its locked case on your way to work cause your going to the range after work. I only know this because I got one. Fun Fact: In NJ, a state of over 8 million residents, there are only a little over 3000 CCW permits and most are held by retired LE.
You can see your CCW and state firearems laws here:
http://www.packing.org/state/