Steering wheel controls with zener diode?
Without a more inclusive schematic to give it context, I'd guess that the Signal- line is actually ground, and the buttons are read by measuring the resistance from Signal+ to ground.  The bottom diode would likely be to protect the backlight from voltage spikes.

The one on top I'm less sure of.  It may be protecting from spikes from nearby circuitry, or protecting from spikes on Signal+ from developing sufficient voltage to damage the switches.  Since it only has voltage on it when no switch is closed, it may just be regulating the "default" level at which no switch closure is detected.