Stock car radio hook up to amp and subs

a stock radio cannot power subwoofers.  even if the stock system had an amp for a stock subwoofer. 

an external amplifier is required to power subwoofers.  this amp should have built-in high-level inputs and auto turn-on so you can interface with the factory radio.  A dedicated power wire ran to the battery and fused at the battery is required for the amp.  the amp needs to be compatible with the sub selection, but a monoblock amp that is 1 ohm stable should be fine.  it helps to understand how the subs are wired (how many connections on each sub) and what the exact model number is.  we're interested in the final impedance load the amp will see.