In my search for a new-to-me G35 I am noticing that some cars have a rear spoiler and integrated brake light mounted on the trunk lid but some other cars don't have that. I searched the forum for an answer but came up empty. Was there a specific package that came with this spoiler from the factory? If so, what package was it and what else was included with it?

On the 03-04 there was a Nismo kit. Different bumpers and sideskirts, 5 spoke wheels instead of 6 spokes and i believe different struts as well. Not sure about the 05-06.

The rear spoiler on 03-04 came with the Aero Package, which was only available on cars with Sport suspension. Nismo bumper & skirts were the Aero Kit, I'm not sure these were factory installed option, I believe they were dealer installed.

^ I could be wrong also but couldn't you get the spoiler without getting the "nismo kit"?

Yes you could. I see lots of non sports running around with spoilers. More than likely it was a dealer option though.

Ahh ok. I guess this is where my lack of on-the-spot knowledge of these cars comes into play. I'm seeing some cars that appear to have the same side skirts but of those same cars, some have spoilers and some don't. I wish there was a way to decode the VIN to know what packages a specific car has. Too bad Infiniti didn't include that in their VIN numbers.

Check out this link for package information.
http://www.g35club.org/download/2004_sedan_specs.pdf
You'll notice there's no mention of the Nismo Kit and as I recall it came out as a kit some time after the car did. I doubt you'll find a 2003-04 with a Nismo kit (painted sides and Nismo bumper), and if you do it has nothing to do with packages, most of these cars have black sides, ugly bumper. If it's a manual it has sport suspension, If it has a sunroof it has the Premium package, and can also have the sport suspension. No sunroof, no sport suspension. If it has red struts and 5 spoke wheels it has sport suspension

Awesome, thank you so much for that PDF! That's exactly what I needed. I may just print that out and use it when I'm looking at new cars to determine what exactly it has!

Now just to get my damn truck sold so I can get my hands on a new G!

my 04 sedan came with a sunroof, spoiler, and sports suspension; however, no nismo/aero package (front bumper, sideskirts, & rear lip) ;/

The 03-04 was the only first gen G OFFERED with the Aero package. Not Nismo. Everyone just calls it the Nismo bumper because of JDM. Infiniti describes it as a 'aero performance kit'.

I still remember the day my dad got his 2004 G35X. They had a silver X sedan on the showroom floor with the 'Aero package' and I remember telling him to get it but it cost too much money for him to justify it.