Relays help to control high power requirements with low voltage power thru 85 and 86 will initiate a connection to allow power from 30 to 87.... There's a couple of websites that help out here

http://www.mp3car.com/the-faq-emporiu ... nd-how-to-wire-it-up.html

http://www.bcae1.com/relays.htm

powering up the 85 and 86 is a cleaner way to control the connection between 30 and 87