The speakers are riveted in so you'll need to drill the rivets out to check. The multiplugs don't usually give much grief so its likely the speaker itself is dead. They're 13cm diameter, if its a mk1 anything citroen peugeot up to 99/00 will have the same yellow plug. After those years its a smaller black and blue one. If you have a scrappy nearby you're bound to find some that will fit in the rear doors of a xsara/406 5 door, or obviously another saxo if you take something to get the rivets out.
Alternatively fit a nice aftermarket set