Making your wire harness
After you have all your parts together, the next step is to "make" your wire harness. You do this by crimping or soldering the harness that comes with the G4 to the one specific for your car. You can find the car specific harnesses on crutchfield.com. I soldered mine, which is harder and takes longer, but makes for cleaner connections.
To solder two wires together, strip both ends of the wire, then slip your heat shrink tubing over one of the wires, then cross them in the center.
Twist the wires together lengthwise and heat the wire on the bottom and apply solder to the top along the length. Use the wire to melt the solder, not the soldering gun. If you are soldering correctly, the solder should wick throughout the wire towards the heat. After the solder cools, move the heat shrink tubing to cover the joint and use a lighter or a heat gun to shrink it.
A more in depth guide on soldering can be found here (http://www.solderinguide.com/).
Your G4 completed G4 harness should look something like this, notice that there are a few wires that are not connected. These are for the parking brake, reverse light input, remote control, etc. that were not on the stock radio hardness. All of these but the parking brake wire are optional to connect, but may reduce funtionality. To connect them, you need to find the appropriate wire in your car, you may have to run wires the length of the car to tap the proper one. i.e. the parking brake wire can usually be found near your shifter and the reverse light wire connected to your reverse lights. Before connecting any wires, be sure you know the what its function is. DO NOT USE JUST THE WIRE COLOR as they usually match, but not always. You should use a voltmeter to test when the wire is "hot". As of when this forum post was made, there are currently no instructions on how to hook up the brown steering wheel control wire and it is currently not supported, but may be in the future. Unfortunatly, the way to hook it up varies greatly by car and we can not currently provide specific instructions on how to hook it up.