Thanks yes definitely after-market... and this is facing front, my guesses were it was a splitter for a sub/amp, most inverters are bigger and have switches and more outputs ? Thought the other side was blocked but will take another look. It borders on home-made looking.
Three most prevalent places to start looking for the antenna are:
1) A mysterious black box in the upper right corner of the windshield
2) on the roof
3) under the third brake light panel at the rear of the roof