iPhone Mic volume

Salivations!

I'm using and loving TrackAddict on my iPhone 6. I wanted to improve the audio being recorded, so I tried connecting a stereo condenser mic attached to my back license plate into the iPhone. I'm using a splitter that separates mic and headphone connections into the single 1/8th inch iPhone jack.

Trouble is the audio is distorting / clipping badly, and I can't find anywhere in the app or anywhere on the phone settings to turn down the mic input. I tried adding a volume dial like that used with headphones, but it has no effect on the mic input.

I have a friend who runs this kind of setup on a go pro, and it sounds incredibly good. So, hoping someone out there might have a suggestion on how to make this work on an iPhone and with TrackAddict?

Thanks a bunch!