The ongoing upgrade of my daughters Barchetta is going well. I've just spent most of the day installing a pair of Vibe LiteAir5 thin shallow speakers to replace the original Fiat ones, which were rubbish. I've also installed a Pioneer TS-WX110A under-seat subwoofer with a built in amplifier (tight squeeze but fits). I've kept the original Pioneer head unit which is in pretty good condition. Does anyone have any ideas where or how I can install a second (rear) set of speakers? I read somewhere that running a 4 x 40w output head unit with only two of the ouputs actually connected to speakers is not a good idea.

No problem hooking only 2 speakers to the head unit. i think 95% of all barch drivers do it like that.