pioneer avh p4100dvd wireing?’s

Gonna be installing a Pioneer AVH-P4100DVD in my 03 Silverado next week. I am confused on the wireing for the parking brake wire. The directions say to hook the wire to the 12V wire at the parking brake switch. It seems to me there will be two wires there to simply complete a circuit to turn the light on on the dash by either supplying the light a power source or a ground. Should I hook the wire from the Pioneer unit to the wire at the parking brake switch that only has 12V on it when the brake is set?  In the directions it shows the wire being spliced into a 12V wire that goes in on one side of a parkng brake switch but the other side of the switch is showing ground, unless they are trying to show a bulb in the picture and not a switch. I don't know I am confused.  Can any one help me?? 

Thanks.

The output of the parking brake is Ground.   If there are 2 wires on the switch, one will show ground all the time.  The other will read 12 volts when the parking brake is not depressed.  That same wire will read ground when the brake is depressed.  This is the wire you need to connect to.  Most parking brake switches have only one wire going to the switch.  It gets ground through it's mounting bracket.

So evein if it only has one wire it will still read 12V until the pedal is depressed?  What does the radio look for? 12V or ground? Or does it just look for the switching from 12V to ground? It just has me confused and I don't want to ruin the radio by hooking that wire to a wire that has 12V if its supposed to be ground or vise versa.

Thanks again.