I just bought a new Chevy Blazer RS and am having the same trouble with the volume on my phone. Two different techs worked for a couple hours to resolve the problem and there is no resolving the problem. No matter what is turned on or off, it stills lowers my volume down to bare minimum. The techs have never seen a problem like this and were baffled. They said I should go into the Apple store and explain the situation to them and see if they have heard of or found a solution to this problem. Going tomorrow to Apple.
I have an Iphone 6S plus. I will update once I go into the Apple store.