HL-50 Is perfect if you want to use your stock car radio but you don't have RCA Output to go to your New Amplifer.
With this Line out converter you will be able to transform a 2-ch Amplified Signal to a Normal RCA Output. You can isolated each speaker output of your car and send it to a Master Amplifier.

2-ch Hi/Lo Level Converter
Level Adjustable
Ground Loop Isolation and Noise Elimination
High Quality Aluminum housing
Max Input: 55W
Dimension: 3.6''(L)x2.6''(W)x1.08''(H