Originally Posted by SolidKatfish

Radio's now a days are smarter and need to detect the parking brake wire being engaged after the radio turns on. You do this buy adding a relay. The radio, once turned on, will detect the parking brake wire has been grounded and will remain locked while driving. With a relay added, the radio with turn on, parking brake will have no connection. After the unit turns on, a signal will be sent through the blue/white wire (amp turn-on), which activates the relay. Which then simulates you pushing down the parking brake after the unit turns on. All this happens very quickly but you get the idea.

Basically when the radio turns on, parking brake wire needs to show no connection first.

I hope you understand it, it's the best way I can explain it. lol.

For Alpine and Clarion screen it's not as simple.