Designing amplified speaker for legacy car navigation system
In the end, I went back to adafruit and worked a bit with their support people:

I used the mute functionality from the navigation system's 10-12 volt input to shutdown the amp, adding a diode to protect the source of the mute signal.  

Note: The SHDN and GRND pins are on AdaFruit's packaging, not the MAX9744 chip itself.







simulate this circuit &ndash; Schematic created using CircuitLab

The values were chosen based on AdaFruit's packaging of the MAX9744 and their schematic.  This is what I wrote on their forum:


  I've learned a lot about voltage dividers -- I was bench testing
  resistor values and found that a value of 10K didn't activate SHDN (I
  really want SHDN, not MUTE, since 99% of the time, this device can be
  off.) I then found the diagram for the ADAFRUIT MAX9744 board. Turns
  out adding another resistor to GRND is essentially a voltage divider
  with a 10K connected to 3.3v. 
  
  The MAX9744 specs say that the voltage has to go under 0.3*Vdd for LOW
  (the internal 3.3 volt Vdd) and over 0.7*Vdd for HIGH. So, I
  determined (using the voltage divider formula and 0.3*3.3v=1.1v) that
  I needed to connect SHDN to GRND with something less than 4.2K. But I
  also didn't want too much current from the MUTE signal, which would
  also be connected to this resistor, so I picked something on the high
  end. Then I picked a value for the other resistor that would raise the
  voltage greater than 0.7*Vdd assuming a voltage around 11 volts (which
  is what I measured from MUTE, and the diode drops another volt.)


I put it all in an aluminum box (also grounding the box.)

I ended up with a ton of audio noise from the car.  I recorded the audio on my iPhone (don't have oscilloscope), put it in Audacity, and noticed what was probably the spark plug noise along with the alternator noise.

I isolated the cause of the noise to the car power supply using an external 12v wall adapter power supply on an extension cord.

I added a 12v voltage regulator (NTE1914) per their recommendation with a 100uf electrolytic cap on either side -- didn't make much of a difference.

I then bought a prepackaged power filter for cars, IBNF10.

The one I received had a 1000uf 16v electrolytic cap, a 1uf 50v electrolytic cap, and a ferrite toroid with about 10-11 wraps:



Noise is only barely audible now.  I am curious, should anyone read this, why the 1uf cap wasn't a ceramic one.  I'd heard that you usually pair a bulk electrolytic cap along with a small ceramic cap in order to soak up the high frequency AC noise.