im new to bass, and im positive that bass's have truss rods, except..mine is no where to be found. its a ibanez gsr200. it might sounds like a dumb question but do some basses not have them?
there is an emblen held on by usually 3 screws on the head it is underneath the strings where the strings meet the neck. unscrew it( the emblum thing) remove it and the truss rod will be under it.

it is usually an allan to adjust the rod
song stuck in my head today