Have both the Metra 70-1786 (w/RCA connectors) and the Metra 70-1784 (w/o RCA connectors). Crutchfield sent the 1784 on the idea that the 1786 RCA connectors did not throw enough juice to kick on the amp for the rear speakers. Not the problem. Sound out of front speakers seems louder and clearer with the 1784. 1784 also appears to be identical to the Scoche discussed elsewhere on this forum.

Absent a fix under the first question, current plan is to simply buy RCA connect speaker wires, plug into the rear speaker connector on the back of the D558BT, and run directly to speakers while skipping amp. Should let the fader work and round out the sound, although I'll lose any thumping (which does not drive me anyway), and a lose a little more gross sound level over the stock HU.