Problem solved, I think.
The JVC h/u has two sets of RCA line level outs, one
pr of full range marked "front" and one
pr for the separately powered subwoofer, which I don't have. (BTW, the JVC has no fader control, so no need for front and rear RCAs.) I cut the ends off the full range RCAs from the adapter harness and wired them up to the other adapter harness connector for the front speakers, which plugs into the brown factory connector ("B" in the diagram below) that has all 4 speaker connections.
Doing this I thought I'd get sound out of the dash speakers only, but instead got it out of all the speakers, including the factory sub behind the seats,all connected to my factory amp. Even with the crap factory speakers the sound is considerably better. Love the HD Radio. The dash speakers are being replaced with Focal 100 CVX coaxials later this week. I'm really looking forward to what I expect will be an additional major improvement. When I have some more time I'll be putting 6.5" Focal woofers in the doors.
Thanks Burg Boxster for confirming what I thought about using the line outs instead of the speaker level outs, and everyone else for their helpful insight. I was expecting a simple plug-in installation with the harness from Crutchfield. This one required a little more thought and know how.
Gary