Harness's ground connection?
 

Crutchfield tells me this about the iDA-x305:

"The harness does not have a ground connection, so you will need to connect your new receiver's ground wire to a grounding point in your vehicle."

Are they saying I won't be able to just splice the metra harness with the HU wires?

You can use the ground anywhere on the car. Just make sure it's connected to the chassis of your car.

I'm not sure how I would go about attaching an HU wire to a grounding point... (I thought I would just be soldering it to the harness that Crutchfield includes)