After installing the 8.4" Sync 3 upgrade radio, you may notice that your Mode button on your steering wheel no longer functions, this is due to the radio screen size being tied to the steering wheel configuration.
There's a 4-inch Configuration:
And an 8-inch Configuration:
As shown above, the 8-inch configuration has a set of arrow keys on either end of the steering wheel, controlling a respective side of that vehicle's EVIC cluster screen. A 4-inch configuration only controls options on a 4-inch configuration cluster and a 4-inch radio. And, due to the fact the steering buttons cannot be reassigned during programming, the only way to keep all functions of the controls after a 4" to 8" Sync 3 upgrade, is to change the steering wheel out as well.