I recently installed a HK Drive + Play 1 in my 2004 Mini Cooper S.
The FM modulator succumbs to the same problems as they all do. Therefore, I want to use the AUX in on the head unit. Does anyone use the factory AUX in connector to the line out jack on the D+P?

I know that the Mini’s head unit needs to detect a load before the AUX mode is available, and because a line out does not provide the same level of output as a headphone jack does, I am concerned that the AUX option will not be available after plugging into the D+P's line out.

 

It should work just fine. I have the OEM aux input in my MINI... and I can still select the Aux input as a source even when there's nothing plugged into it.

The aux input was designed for and works best with a line out from a device. Using a headphone jack is not ideal in any situation, because that would be double amplification (amplified once from the device, and again in the head unit).