Audio Systems: radio wiring, butt connectors, radio wiring

Question
I was replacing my factory system with a newer AM/FM CD player system and cut the wire harness leading to the vehicle. I have tried using the butt connectors to splice the wires , but find that they do not hold the wires together well at all. What else can I do?

Answer
Use a soldering iron and put heat shrink over the wires before connecting them, Then, once the soldering is finished, you can slide the heat shrink over, and heat with a heat gun.
Or, use the correct size wire splices, probably red. And a racheting crimp tool