Line Level to RCA Adapter Help

i know that this can not contain many components, but what do you need to convert speaker level to RCA signal for a car audio system. i know that they sell an adapter, but i don't have room for a large box behind my head unit. i wanted something that i could build and heatshrink on a small PCB. any help is greatly appreciated.

Two resistors will attenuate a speaker level to line level for each channel.
The values for the resistors depend on the power output of the amplifier.