The harness supplied by Crutchfield plugs directly into the original plugs. Simply splice the harness leads to the HU leads and you're off. I soldered the leads so I don't have to revisit them later. But in the past, I've used bullet connecters and a crimper. I've had good luck with those as well. My goal in any aftermarket mod is to be able to revert if I get ready to sell/trade. I can get more for a HU on eBay than a dealer will give me on trade.