for wireharness i solder, but doing remote wires and others, t-taps are your friend. i cant see myself doing audio install these days without t-taps and spade connectors, other terminal connectors. work goes by so fast and reliably.
something like this. i use red and blue depending on gauge.
Amazon.com: t-tap red
Amazon.com: 3M Scotchlok Nylon Fully Insulated Male Spade Type Connectors 22-18 Gauge (Red) - 100 Pack: Car Electronics
Amazon.com: Ratcheting Crimp Tool For Insulated Terminals: Electronics