How can you tell if the speaker in the speaker in the back of your 1996 Eddie Bauer Explorer needs repalced?

If by 'speaker in the back' you mean the subwoofer then it is easy to tell if you need to replace it. First, remove the subwoofer panel, be carefull at it since it is very tight. Next, inspect the cone where it meets the outer structure. The original cone is paper so humidity and time along with use sometimes separates it and this will cause a 'blurred' sound instead of a well defined bass sound. I personally inspected the subwoofer inner stuff and it has some cotton-like vibration-absorbent material.This is a very efficient design. Check your cone integrity and replace as needed. A lot of people just upgrade their trucks with aftermarket subs. This also aplies to rear door speakers since they're usually larger than front door speakers.Good luck!