Thanks to everyone's help I did it! The empeg is now politely letting my GPS tell me where to turn.

I used a voice activated switch kit (only $7, cool) I found here -> http://www.rainbowkits.com/kits/vox-1p.html and with a few minor modifications it works great. I used it to find my way to work this morning (need all the help I can get in the mornings). One of the modifications I did was to replace a resistor (R2) with a potentiometer so I could adjust sensitivity. Once I tweaked the pot a few times the VOX switched great. One thing that helps out on the timing is that the GPS has an alert tone that beeps right before it starts talking. The VOX switch activates on this beep so it doesn't cut off any of the voice prompts. I also noticed that the mute line on the empeg is way more 0 volt sensitive than 12v. Instead of trying to drive the mute line to 12v I used the vox to drive it to 0v.

The Hijack mute/button hack to switch AUX on and off worked great. So for a grand total of about $15 (and a few evenings) I've got an integrated empeg to GPS solution, I'm pumped.