Help, my 2008 Alfa Romeo 159 Jtd has odd light behaviours (all the light in my car include cabin light, head light tail light etc darkens and brightens continuously with a sound). I think it's the alternator, what do you think?

often it is, but find a place like auto zone where they load test batteries and alternators free . It could be either, both, or a wiring short causing all that.