Does anybody know what the slotted dial is for on the tuner module?

Identification number.

In theory, this would allow you to have more than one sled installed in different cars, each one with a different ID number. Then the player would remember its settings (EQ, loudness volume, fader) differently for each car you inserted it into.

However, I do not know if this feature was ever implemented in the software, even the most recent software. Does anyone know if it's in there or if it is supposed to go in there by 2.0 final? I'd like to FAQ this one if I can get that information.

Also, does anybody get ignition whine through the tuner only?

It is possible that your tuner might have a wiring problem such as the one described here. Or it could be something wrong with your antenna wiring or the power antenna wiring, as you theorized. It could also be some kind of RF interference being picked up by an unshielded antenna. And of course, the antenna connection is simply one more way to potentially pick up a ground loop.