sounds like he wired the amp to the switched 12v, white wire/ red tripe, this should be fine unless you only want the amp on when the hu is on. your car doesnt have a power antenna so you don't need the blue/white wire and you're already getting power to the amp so you don't need the blue either, just wrap up those and the ones in the factory harness.

as far as connections go: solder and heat shrink is the preferred method, then solderless connectors/crimp connectors. twist and tape is fine if you know youre going to change something in the near future, but for a long term thing it's not so great.