Welcome !,what trim was your friends car ?,because i think the lower trim levels didn't have the amplifier behind the glovebox,hence why the volume is very low,or the other reason could be someone fitted a by-pass cable on the amplifier,that's why the cheap cd player worked ok.If you take the glovebox out and look towards the pillar,the amplifier will be a plain silver unit (not marked like an aftermarket amplifier) if it's there i'd see if the original loom is there and plug it all back in and remove the by-pass loom,if it's not there you have two options,fit the original plugs back and get a head unit from a non amplified car or just fit a better aftermarket head unit,HTH.