Line out for iphone 6 dock

So I use an extension cable connected to a iphone dock to get line out for my car.
The new iphone 6 lightning dock has a lot better charging than the 5S dock, but it also allows both line out AND headphone. When i connect to it, it allows me to adjust the volume, which is kinda a pain because I never listen to the headphones at full volume.
So how can i get it to use the (im assuming) locked line out.
Im thinking I might be able to put some resistors on the line out cable to convince it its connected to an adjustable source.
Would this work?

Informational Link
https://support.apple.com/en-us/HT202783

In case anyone comes across this:
the new dock lists that it supports 2.4A.
I did a test with an iphone 6 and a 2.4A charger with lightning cable.
Charged from 68-73% in 10 minutes on the 5S Dock (with extension cable)
Charged from 73-83% in the same time on the 6 dock.
However, the new dock allows you to adjust the volume of the output via rocker switch so its less effective at 'line out' even though it says it supports both...