Since I'm going all-linux for this build, I needed to figure out how to mix in the audio from the line in with the computer's audio so that I could have my radar detector play through the car speakers with its volume controlled by the computer's master volume.

Solution is in this article I found:

In case it ever goes down,

To enable it, you run this command:
pactl load-module module-loopback latency_msec=1
To disable it, you first run this:
pactl unload-module 27
Find the index number for the module ex:
    index: 22
	name: <module-cli-protocol-unix>
	argument: <>
	used: -1
	load once: no
	properties: = "Lennart Poettering"
		module.description = "Command line interface protocol (UNIX sockets)"
		module.version = "0.9.21-63-gd3efa-dirty"
22 would be the index.

Now run:
pactl unload-module 22
Where 22 is your index.

To enable this permanently, add this line to the end of the file: /etc/pulse/
load-module module-loopback