A3 audio problems please help

I bought a 2002 audi A3 (8L2) 1.9tdi sport (130) 6spd,

no standard audi concert, tho is equipped with half amplified system, I fitted an old unit i had lying around tho when I connected the iso connectors only the front worked? I found a set of phonos already tapped into the loom and found the amps power wire to get the rear speakers working, rear is delayed and doesn’t sound as good as the front? Is this the norm this way? Is there anyway of making it sound better? Or even hear the sub? Audis normally have good sound systems?

any help and advice would be appreciated