I go over a standard 16 pin harness and tell you all the functions in it. applies to: Car Stereo and Marine stereo systems, Wiring Explained In Detail.
Usually, it is best to make all of the new stereo's wiring connections via the wiring harness, but if you have to make a direct power connection, you'll need to know the difference between "switched" and "constant" power:

A switched power source is only on when the ignition is keyed. Connect your new stereo's main (switched) power lead – usually a red wire – to a switched power source, so that the stereo will turn off when you turn off the car, and not drain your vehicle's battery.
A constant power source is always on. Connect your new stereo's memory lead – usually a yellow wire – to a constant power source, so that you don't lose your radio station presets, tone control presets, and clock settings every time you turn off the vehicle.

Although rare, a few highpowered stereos require you to make a direct constant power connection at the positive terminal of your vehicle's battery. This requires a heavier gauge power wire, an inline fuse (usually included), and a ring terminal to connect the power wire to the battery clamp. You will have to route the power wire to the battery location, which is often through the vehicle firewall and into the engine compartment in order to make the connection at the battery.
Speaker wires

Car stereos have eight wires for the traditional 4speaker system – a positive wire and a negative wire for the front left, front right, rear left, and rear right speakers. Depending on the wiring configuration in your vehicle and the wiring harness adapter we offer, some of these may not be used.
Ground wire

A good ground connection is vital for proper stereo performance and to eliminate unwanted noise. If you are not using a custom wiring harness, look for a bare, unpainted bolt or screw that contacts the bare metal of your vehicle's chassis. Loosen the bolt, slip the ground wire underneath (this is almost always a black wire), then tighten the bolt. If your ground wire doesn't contact bare metal, your stereo won't operate. A loose or weak ground connection can result in signal noise interfering with your music.
Indash video — tapping into the parking brake wire

If your new stereo has a touchscreen or video monitor, you will also need to connect a wire to your emergency/parking brake wire. This wire acts as a switch to turn on the video monitor when the parking brake is engaged.

This wire can be in different locations in different vehicles, depending on the brake configuration. The wire is usually found where your parking brake is. In vehicles that have a hand brake between the front seats, you'll have to remove the center console to get to it. In vehicles that have a footpedal parking brake, the stereo's wire will need to be routed to it under that dash. Either way, it isn't too hard, just take your time. And once again, awardwinning tech support team can be a big help in locating it and helping you get to it.