Mitsubishi L200 electrical issue, new battery and alternator installed

First thing that happened in the lead up to this was the stereo died (figured old crappy stereo) but then the central locks stopped working, then I noticed the battery died, replaced the battery (still under warranty) no luck! So replaced the alternator and belts! Still no luck! Now I'm beyond my knowledge and desperately need to get this solved myself as no cash for a mechanic.
I have a voltmeter but need to know where to start!