The blue/white wire from that specific pioneer harness needs to connect to the blue AND the blue/white on the metra, auto antenna would work then.
Verify your metra has a blue and blue/white, in pins 3 and 13, or a jummper, and connect them together and then connect that to your blue/white remote on that deck. Some pioneer harnesses have both a blue and a blue/white.
There's a small antenna adapter plug and a Y harness to use both antennas if reception is lacking. -
Scosche Mda-B Micro/Delco Antenna Adapter Tip
METRA 40-UV43 MALE TO FEMALE MOTOROLA ANTENNA CABLE
JerryB, great insight as always. The Metra harness DOES have a blue wire, but I left that unconnected as I didn't capture this from the FSM wiring diagram. I'm going to pick up that antenna plug/adapter, pop the stereo panel back out, and connect that blue wire along with the second antenna. Thanks!