Ok, let's look at what we just did. Pins 85 and 30 were both hooked to constant power. When we open the door, it sends (-) power to pin 86 on the relay. The (-) power now trips the low amperage side of the relay and sends your constant power to your new lights, turning them on.

Pin 87a
Pin 87a is sometime called the "at rest" pin. When the relay is NOT triggered, power from pin 30 goes to pin 87a. This can actually be used to do some very creative things. Here's a really cool example. Most newer cars have a buzzer that'll sound if you leave your headlights on after removing the key from the ignition. Many older cars don't have this feature.

This example will use pin 87a to create a headlight-warning buzzer. The first thing you'll need is a small 12v buzzer. You should be able to pick one up at Radio Shack.

First, mount the buzzer under the dash and ground the (-) wire from the buzzer. Now connect the (+) wire from the buzzer to pin 87a. Use an electrical tester to find the parking light (+) wire coming from your headlight switch. Connect that wire to pin 30 on the relay. In this case, the parking light wire can power our buzzer because it draws so little power.

Now ground pin 86 on the relay and connect pin 85 to an ignition wire. This is a wire that has power ONLY when the ignition is on. Now, if you have the car turned off and the lights on, the buzzer will sound!

Let's look at how this works. Pin 86 is grounded and pin 85 is connected to ignition power. When the car is turned on, the relay will be triggered and the contact will be sending parking light power to pin 87. In this example, no wires are connected to pin87 so our buzzer won't sound.

When you turn the ignition off, the relay is no longer triggered so the contact flips to pin 87a. If the parking lights are on, the power from the parking lights will power the buzzer and you'll be reminded to turn off your lights!

You could even take this example one step further and hook the (-) side of the buzzer to the (-) pin switch on your door. This would cause the buzzer to sound only if:

Your ignition switch was off, and
Your lights were on, and
Your door was open!

Cool huh?