Hi @brandon_k ,
For what it’s worth, here’s the wiring diagram of the audio unit for a 2006 F-150.
(click on image to enlarge).
Hopefully it is of some help.
There is useful information in the wiring diagram especially when coupled with the head unit user manual.
It shows the 5 connections needed to make the subwoofer work and where they're connected to on the sub woofer. The head unit user manual shows how to connect a subwoofer to it.
The subwoofer battery and ground connections are on 5 & 2, the audio inputs are on 7 (+ve) and 8 (-ve) and the enable lead (sometimes known as amp on) is on 1. Hopefully they are marked on the subwoofer unit as to what each connection is or at least there is a numbering system.
Assuming that your head unit is a Pioneer MVH-X370BT as linked by you, then here’s an image from the user manual showing what wires/terminals the sub woofer needs to be connected to in the head unit. If this is not the correct model, check what it says on the head unit regarding the model number - you have to remove it anyway to access the harness wires so maybe it is on the back of the unit, and then search online for the correct user manual as usually they have the wiring installation details.
(click on image to enlarge)
The battery and ground for the sub woofer need to be a separate feed. I suggest that the battery be fed via an inline fuse, direct from the car’s battery terminal. Ground can be from any suitable clean chassis point. The wiring diagram also shows that the subwoofer requires a 25A fuse.
The subwoofer audio +ve and -ve input connections from the head unit may depend on the subwoofer’s impedance, i.e. whether it is 2Ω or 2 x 4Ω (see image above whether to use connection 18 or 19). Hopefully there is some info on the unit which may tell you.
The enable wire i.e. the blue/white wire from the head unit is #17 on the schematic.