Sorry I didn't post sooner -- I removed the mic / map light assembly from the headliner yesterday and cut some sound insulation foam I had lying around (part no. 5692T13 from McMaster-Carr). I used the plastic assembly and made a T-shaped template and cut the foam then reinstalled with the foam in place. I haven't had a chance to test yet, but will test it this afternoon (have a 3-4 hr drive). I'll post back tonight as to if this makes a difference. If not, I suspect it's the omnidirectional mic picking up all the ambient road noise in the car.
The microphone itself is a small package, but there is a significant amount of space between the microphone and the car skin -- I feel confident a directional mic could be placed in there if need be. The microphone is wired exactly as I suspected, 3 wires -- I'm guessin a power (probably 14.4V, but I didn't check), a ground, and a signal -- it shouldn't be too difficult to find a replacement mic to wire in.