Is it illegal to have an empty Nitrous bottle in the car not hooked up to the motor... Like just in my rear deck for show...

 

The law varies by state.

 

Do a search in this section. there is a good thread, several pages long that discusses several states.

 

SEMA's state-by-state summary of "legislation pertaining to the motor vehicle aftermarket", including nitrous: http://www.enjoythedrive.com/content/?ID=25259
SEMA legislation-related announcements: http://www.enjoythedrive.com/content/?id=7984

Laws from the 'controlled substance' side: http://www.cognitiveliberty.org/dll/N20_state_laws.htm

LS1Tech law threads:
https://ls1tech.com/forums/nitrous-oxide/195341-nitrous-laws.html
https://ls1tech.com/forums/nitrous-oxide/205051-court-over.html
https://ls1tech.com/forums/nitrous-oxide/15635-texas-nitrous-laws-ii.html
https://ls1tech.com/forums/nitrous-oxide/15615-nos-illegal-use-laws.html
https://ls1tech.com/forums/nitrous-oxide/15334-nitrous-legality-anyone-know-laws.html

Happened to find a city (Hubbard, Ohio) that actually has its own local laws against nitrous:
http://www.cityofhubbard.com/Ordinan...1/page0064.asp