I have a similar fault with mine in that it has lost all sound on drivers side and having researched it is my head unit which is at fault. I checked fuses which were fine, I then removed the head unit to check wires, which is an easy task (click on following link or read below).
1, Open flap in front of cassette player, 2, Pull volume knob away from radio and you will see a small screwdriver slot at 12 o'clock where the knob was. 3, Using a thin, flat bladed screwdriver, turn a quarter turn anti clockwise and withdraw the unit. This will allow for checking the wiring behind. ( I checked all wiring to speakers as well and tested the speakers as well) Apparently, the standard business radio cassette is known for this problem as well as pixels going missing. There are people offering a repair service for sound problems on ebay at £100. If you decide your head unit is at fault and you want the same replacement, bear in mind there are different types of unit which all look identical and both types cost between £60+. Main thing to check is pin configuration at the rear as some early models have round pins and later models flat pins. Some also have a power amp in the boot alongside the cd changer which can also go faulty. There are some on ebay both flat and round pin. There are also standard fit radio units with adapter plates usually about £120+ and have all the necessary wiring adaptors with some even being able to hook up with remote control on steering wheel. One more option is double din ones with satnav and TV but are the most expensive at £300+. Hope this has helped and you get sorted soon. ;)