Noticed the same thing here haha... I was wanting to hear how you adjust the volume on the different speaker zones... Has to be some sort of FADER thing.
My trailer uses the JENSEN car radio and its two speaker zones for inside and outside is a slide lever for the front and rear car type speaker audio levels. Having the slide lever in the middle gives same audio levels for both speaker zones.
I realize all speaker setups are different but surely you have some method of controlling each speaker zone audio levels.
Check page 15 in your user manual for adjusting audio level on ZONE 2 speakers...
You can download the user manual here if you don't have one