RED light with battery visit workshop on 2006 Mercedes-Benz. Aux battery?

Yes. It's imbedded in the speedometer, graphic section. It says "Visit workshop" with a bright red battery below it.

The red battery light indicates that there is a malfunction in your vehicle's charging system. This would mean related to the alternator, voltage regulator, wiring and cabling, or even the drive belt for the alternator. This is a pretty serious issue, as typically when the light is on, there is no charging of the battery occurring. Also, your vehicle requires a great deal of power to keep various components running, all which will be compromised quickly as the battery depletes. Have this checked right away, or you run the risk of stalling out and having the vehicle become disabled.