To add an amp to the factory system, the best place to get the signal is from the radio, not the amp. It will be a lower voltage output, which will be better for the amp.

Heres a better solution, but I have not reied it with our radio. The crossfire radio actually used Blaupunkt plugs on the back. Go to a blaupunkt dealer and ask them for the RCA out connector for a blaupunkt radio, and plug it in. I had a base model radio, and i know that the factory system was not using this plug, but I don't know if it is working on the factory radio. If the plug is being used, reference the blaupunkt connector for the pins to use to solder a set of RCA's to the REAR outputs. If none of this will work, buy a high-low converter to conver speaker level to low level RCA for an amp to use.