Okay, here we go. First, acquire some thick wire (12 gauge would be good). Great wire for this would be an old two-conductor extension cord. I had some black-and-red 14 gauge wire lying around, so I used that. You will also need a vehicle socket replication. I found mine on some GameBoy accessory, of which I had to remove the 20 gauge wire. I also like the All Electronics Corp. who have some great parts at good prices (allelectronics.com). The only thing I was able to find there was one with sockets on both ends of a wire (so you could have two!) at www.allelectronics.com/make-a-store/item/CLP-94/20-CORD-W/-CIG-LIGHTER-SOCKETS-BOTH-ENDS/1.html
for $5.70 at the time of writing. Depending on what you chose to do, you will also either need wire nuts or crimp connectors or solder/heat shrink tubing. You will also, of course, need your trusty soldering iron and any screw drivers that your car may require. Usually, a Phillps-head is good enough, but Torx are also very popular in vehicles.