The only way I know how to do it is just to leave the ignition on. If you spliced into the factory harness and ran a 12v constant line into the radio power it would probably mess with the CANbus. Not positive and I'm sure someone will chime in here, but the radio set up on Chryslers is very different since 2007. That is why the aftermarket harness to retain the radio on after shutoff function is over $100. If you don't care for that function you can circumvent the problem pretty easily with an aftermarket deck. So your options are pretty much wire an aftermarket deck to a constant power source or just leave the ignition on with the factory deck. The door chime goes off after about 30 seconds. Just make sure your fan is off also to save battery.
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