Playing phone's Pandora through OEM radio AUX input - No Go

My bike is a 2013 R1200RT with the OEM radio. I also use a Zumo 660. My Samsung-SM-J320A, which is BT paired to my Zumo. My Sena is also BT paired to the Zumo. Those both work 99%. When I rarely listen to the RT's radio (FM, etc) it's using the bike's speakers.

I used to connect an older iPod Touch to my bike's USB/3.5mm jack under the dust cover on the right side of the "dash". Worked like a champ when I switched the source to AUX. This was back when I was NOT using my Zumo 660 (I had an old Garmin/BMW NAV III from an earlier RT).

Today I thought to myself it would be neat to listen to Pandora using my Samsung-SM-J320A smart phone. So before I went to the garage I decided to do a quick test. To test that the phone's output was working, I booted up Pandora, then connected a stereo headset to the phone's output jack. Headphone worked fine.

Then I took my phone outside, plugged a male-to-male 3.5mm stereo cable between the phone's output and the input under the bike's dust cover. I switched my radio's source to AUX. Unfortunately I heard nothing.....quiet as a tomb.

Any ideas what I am doing wrong. From a few other posts here is sounds like what I am trying to do is correct. Is there maybe some impedance mis-match? Or am I missing something blatantly obvious....wouldn't be the first time.....

Thanks for any help!

PS - I also posted this on another motorcycle forum to get wider distribution.