Your concept could be workable after you do some testing and troubleshooting.
Following are items that need attention.
* The diode bridge has a volt threshold of 1.2V. The audio signal must exceed that level if it is to get through at all. At low volumes it will not get through.
* Pull-down resistors should be attached to the Clock pins of the 4017's. A value between 10K and 1 meg is suitable.
* Both of the 4017's ground pin and clock enable pin should be connected directly to each other. This makes 4 pins that can be connected. You can keep one diode between vehicle ground and the circuit board. (Typically the diode is installed between vehicle positive supply and the circuit board.)
* The output pins of the 4017's need a current path to ground. It may or may not be adequate to use the ground wire of the RCA leads. The diode intervenes.
* Output current from IC 2 pins is able to go into bias terminals of NPN transistors, and down through the LED's. This will light the LED regardless what is happening at IC 1. Reconfiguration of the transistors will be necessary.
Good luck. You show potential.