I have a 09 gl450 and the push to start has been acting up possible not recognizing the keys. I changed the batteries in both remotes and same problem. if i insert the key into the ignition it works fine.
On a side note the push to close button for the lift gate doesn`t work properly most of the time. Note sure if its a different problem. I haven`t noticed a aux battery in the car but it does have a newer battery 6 months old at best but these problem just started within the last 2 weeks. Any ideas

 

Aux batt won’t have anything to do with this, but it lives just to the right of the front half of your main battery.