Radio envy here. SteveAlmost all modern amps have optional speaker level inputs. You run the factory speaker outputs right into the amp. See the yellow and black pigtail below...
https://www.crutchfield.com/S-VVPFwR8KIQU/fg_120_AG_General_Features|YCSpeaker_Level_Inputs/Speaker-Level-Inputs-Car-Amplifiers.html?&pg=1