Also, I would say that a tidy car is less suspicious than a messy backseat. If you want something good to place on the backseat, try the bible or a mainstream news publication or something. keep your car not obsessive compulsive clean, but keep it tidy.

Other items that are good in subtlety : centrist/right leaning political slogans on stickers like republican campaign stickers, NRA, Army/Navy, US Flags, support the troops, ect. Most police are supporters of the republican party and also the military and consider themselves patriotic and these kinds of symbols cause empathy in police officers.

Keep your shit wrapped airtight and in the trunk somewhere. Don't be an idiot.