It is a harness that comes with the aftermarket Blau's. 4 RCA's & the amp turn-on. Apparently, it won't work correctly with the OEM Ford Blau, though. [mad]
I have a harness with a yellow connector that came connected to the back of the HU, but I haven't been able to track down the other end yet. I took apart the left-side carpeted panel in the cargo area, hoping to find sub amp wiring preinstalled. No dice.
I really like the stock MP3 HU, and want to keep it, but I'm not a big fan of line-level convertors. I even thought about soldering in a couple sets of preouts at the HU amp, but after taking the HU apart (I'll deny having done that later), it would be impractical at best.
I'll either stick with a mono sub amp, wire in the preouts, go with the converters, or go with an aftermarket HU. Haven't decided.