With just a few resistors for each speaker channel it looks like a pretty standard passive LOC (confirming I think Naviguy's theory)

As the double din unit I am fitting has 4v pre-outs I think I'll bypass the LOC and connect them to the Rockford amp. My only concern is that with 4v pre-outs I might be able to overdrive the amp/speakers and blow something up!
If I'd known for sure that all the "black box" was was a high to low converter, I would have got the 105b harness and saved myself $4.00 and some work!