Fountainhead

Hi Alan,
could be your 3.5mm cable isn't right for the pinout of your Nexus 5 output jack. Just a WAG not knowing anything else about the Alpine Aux input pinout. Since your Nexus is not grounded to anything you shouldn't need a ground isolator? If you have it plugged into a charger try removing that then plugging into the Aux input.
I know iPhones have a 4 pin headphone output, 1 is mic, 1 is ground reference *not chassis ground but a null reference point, and then L & R signals referenced to that null point, which is a ground reference to the L, R, and Microphone input but not to the chassis.
Your Nexus I know has a similar output configuration with the mic and ground reference pins swapped ( in relation to the iPhone pinout). I would google that and see what cable you need. I know on most of them the typical L, R, Gnd 3 pin configuration type 3.5 mm cable usually works.
good A-B test is to try another phone, any of them should work with a L, R, Gnd, Ring tip sleeve cable.