2010 E350 4Matic... 155,000 mi., 2nd owner (bought in Jan. 2022). All MB recommended services completed. In sport mode, I get a smell not present in comfort mode. Smells like transmission fluid. Thoughts? Also, the dreaded "Auxillary Battery Malfunction" present. Do it myself or take to dealer? Lastly, it has a whine while in motion. Sounds like from passenger side engine. Maybe transmission or 4Matic related? I took it back to the dealer after I bought it. He replaced driver's side wheel bearing. Not the same kind of noise. Thanks for any help you folks can give! It is most appreciated! (Now, can ya help me with my possessed by the devil Passat? Lolol!)

Can't comment on the trans fluid smell you're getting because I've never encountered it on my '11 E350 wagon while in sport mode. The aux battery I can comment on as I had to change my aux battery after getting the same warning on the dash. My aux battery is located in the driver's side dash below the speedometer housing. There is a panel on the left side of dash that I had to pry to access the aux battery. Watch the YouTube video that is attached.
As to where to get the battery, FCP Euro has the OEM battery and it is covered by their lifetime warranty.

Awesome! Thank you for your help! The video helps immensely. I think I can change it on my own. Locally, we have ONE MB/German technician. I have had problems getting my Passat serviced. There is 1 VW dealer within 30 or 40 miles. The MB dealership is an hour away. Beyond that, no dealer within 2 or 3 hours ofy location. Suppose I should have thought of that when I bought the car(s)! Lol! But thanks, again!

Very easy to change, but you may need someone with small hands. I have medium sized hands (8 inch width from pinky to thumb), and even with the plastic side cover it was pretty cramped.