Head cleaner CD. And/or get an aircan and blast it with compressed air in the slot. Don’t use one that’s propane though, I did and it leaves goo everywhere. Blast before cleaning heads I’d say.
To replace, you’d want a double-DIN unit. It can be tricky to integrate it properly into the car, so that the backlight comes on when you turn the lights on and the thing turns off when you turn the car off etc. You either buy a module that integrates with the car’s CANBUS system (which I did) or you take the dash panels off, hunt around for a suitable wire that powers something else that displays the off and on characteristics you want and use that. Oh and you need an adapter for the aerial too, it was a game finding one that worked correctly for me but it’s actually a simple one you need in the end.
Your stereo is an RCD300 – lots available on eBay, and those will simply plug in in place of your old one. You could upgrade to an RNS510 however, there are also several third party replacements that look and behave just like an RNS510 but are cheaper.. 🙂
Or I could upgrade to an RNS 510 and sell you the Pioneer satnav unit that I installed instead 😉