Check the board for items relating to cleaning the tuning mechanism (though that's probably not it)--note that if you have one of the SC-series radios, it will be a different deal from the CR-series.
If you do replace the speakers, check the ohm rating first; if the dash speakers are on a separate set of channels, they will be 4-ohm; if they are in parallel with the door speakers, both sets will be 8-ohm. It will likely be marked on the back of the speaker magnet. Replacing an 8-ohm speaker with a 4-ohm can cause a frying sound and possibly smoke from the head unit. Not recommended. Let us know what you find/do, especially if you find a set of replacement speakers that is a good fit and quality. Note: Crutchfield's isn't always right, but it's a good place to start.
--
We have met the enemy and they is us. [Pogo] S70 cop car : Rough Rider tires& suspension, Walmart fog lights, eBay speakers, ambiance by Pall Mall, trim by Le Duc d'Tape, 8-channel THD by OEM amps