So I recently purchased a 2008 Nismo 350Z and installed a double din as well as new door speakers and tweeters, my question is whether or not I should have speakers behind the seats or are those only on Bose equipped cars because there is no sound coming from the back. Also if there is not any speakers there are there existing cables there to install them.

 

All Zs have functional speakers in the upper panel above the storage compartments.

 

As said above the wiring is there. The question is do you need rear speakers?. We have several threads about that topic.
The most popular option is to get some good components in the front and forget about the rear

 

So I imagine that mine are probably blown then since there is 0 sound coming from them, sound is pretty decent now but I just think it could be a bit better with the extra 2 behind the seats.

 

If you installed a new DD headunit, did you properly plug in the rear speakers? There's also the possibility the previous owner disconnected the rear speakers if they wanted a clear front soundstage.

 

When I still had stock headunit I was playing with the sound settings and if I put the balance all the way to the rear there was nothing and yes I did connect all the wiring up front but just figured there was no rear speakers so didn’t even bother to open up the back.