Could i buy a '15 nav card for my '13 brz?
 

will it work? because i purchased my brz without a nav card

It didn't work for me when I tried on my '13 BRZ. Mine also came without nav card. Dealership charges something like $300 for a new one, and no guarantees it'll work (cause it didn't for me.)

Also your OEM head unit will reset itself if the battery is disconnected and won't start back up without the nav card. You're better off buying a better head unit.