Adding an amp to a factory HU?

I own an '02 Nissan Frontier with the stock Rockford Fosgate head unit. Last month I read an article in a car audio mag, where an install featured a 911 Turbo with the OEM head unit intact. They said it was accomplished by soldering female RCA adapters onto the factory wire harness which fortunately had low-level ouputs. I was wondering, since the stereo in my Frontier has a stock 8" Rockford sub, if there is a way to test my head unit for a low-level signal to connect an aftermarket amp.

try a connect a small test speaker up to the positive and negative for one of the 4 channels.  If you get sound that's barely audible even with the volume turned up, it's a low-level connection.  If it's normal volume then it's a high-level connection. Ethan

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Or just use a speaker level to RCA converter.  They are pretty inexpensive nad easy to install.  If you do use a converter, I would suggest installing it after the factory amp.  Good luck.