I de-soldered the red/black wires and connector from the stock speakers, and then used butt connectors to connect to the white wires with spade connectors that came with the speakers. While I don't remember whether black = big spade or black = small spade, I do remember that I connected the black wire/connector from the stock speakers to the white wire with the black stripe in the Pioneer kit. Hope that helps!
Can someone take a picture of their properly-connected oil pressure sender?
Also, are there two plugs on the sender itself that interface with the harness?
Edit: @benjrblant was a rockstar and texted me a photo of his sender:
Which confirmed what I thought, that there were two connectors going to the sender - which then prompted me to go dig around on my sender and lo and behold, I found the mysterious 3rd plug:
So now to figure out how to disassemble the plug and get the pin out, but I'm betting this thing will work once re-soldered! Documenting this in here for the next poor soul that has to puzzle all this together.