What rhymes with backseat?
mathlete, athlete, tracksheet are a few :)
I think 'backseat drivers' offer unsolicited advice b/c they are fearful. The 'backseat driver' prefers to be in control (in one case I know, she has experienced disappointment from nearly everyone important to her). The backseat driver does not trust the driver and feels a need to let the driver know how they should be driving the car.
A backseat driver is someone who keeps says things like: Watch out for that car! Be careful of that kid on that bike! He is telling you how to drive when you already have seen those things and are doing what you need to do. The term can be used even when you are not driving but someone keeps giving you unwanted and unneeded advice.