The ferrite core supresses EMI interference. CPC, Rapid electronics, Ebay, will do fine.

Alternator wise, it depends if it's just radiating cack but giving a clean output, or whether the output is noisy too. You could get it on a scope to see what the output looks like, if you know anybody with one. Sadly most garages or auto-sparks don't own one.

The three things I'd try are:
- Ferrite as above
- Run a direct earth from radio to chassis (standard wiring goes all over the shop)
- Put a bypass cap in parallel with the radio 12V and ground. 1uF electrolytic will do fine.

Bear in mind though that the 5000 might be suffering from a design flaw or a fault - you may find the issue goes away with a new radio!