Audio Systems: Low radio volume, toyota corolla, radio volume

Question
I have a 1993 Toyota Corolla with OEM radio and 4 speaker system.  I can barely hear sound from the rear speakers only (not the door speakers).  The volume control does not work.  On occasion the sound will suddenly come up.  When this happens, the volume control DOES work.  Then, just as suddenly it will go back down to minimum and cannot be adjusted.  Any ideas or suggestions?  Thank you for any help you can provide.

Answer
Larry,

 since the unit and the speakers are over 10 years old now, i would say its just simply age and use that has affected them , most OEM decks and speakers have a life around 5 - 7 years under average-moderate use and abuse, so yours have done good, but before you dive into a new purchase of head unit and speaker, remove the back speaker and wire it into a simple radio or home stereo to see if it functions and responses.. if it doesnt try another speaker in its place in your car to see if it works, if that one doesnt, i would use a jumper test meaning pull out your radio go to www.streetdreams.com/wires/ to get the color codes for your wires and splice into one set of the wires that work * the front right or left* wire in a set of speaker wire and then run them to your new test speaker, if it works at the unit but not at the mounting location , your unit is fine , all you need is to run new speaker cable from the back speakers to the harness at the radio .. now if it still doesnt work , its time to upgrade to an aftermarket unit , which should fix your problem ... be sure you get a wire harness adapter so everything mounts up nice and easy .. hope this helps you out if you need anything else let me know

thanks