Been a while since I logged in but saw your post. This depends on the model of your stereo which you didn't specify. If it's the RCD-510, you can find the pin assignments on the web. My amp accepts high level inputs so I didn't need a line out converter. I purchased an adapter cable from Enfig Stereo that first plugs into the head unit, then the factory harness plugs into that. That way I didn't have to splice anything on the factory harness. The adapter has pigtails, with 8 speaker connections ready to terminate of your choosing. I used the rear speakers outputs on the pigtail as it was recommended for me. My stereo sounds great for a factory head unit.
Also, you can code the outputs for line level via VCDS, but I didn't need to do this either.
Very interested in this idea of coding the outputs for line level via VCDS. Curious, how many pairs (channels) of line level outputs are there? Are there 4 channels - ie LF, RF, LR, RR , so I could use a 4 channel amp. Then use crossover circuits to distribute the correct frequencies to the correct speakers?
Also, anyone have a schematic or a pin out diagram?