The source is a Garmin Zumo 550
hardwired to unswitched power and mounted to the dash with a RAM mount. It's loaded with maps of Mexico from Bicimapas
The Bicimapas are good but certainly not perfect. The good news is that they include a *lot* of the small dirt roads I like, plus street-level data for most significant cities. The bad news is that the maps are sometimes wildly wrong and you have to be very careful to doublecheck the autorouting. Still, without the GPS my path would be constrained to major highways or "right hand rule" exploration of the maze of Mexican roads. The Zumo+Bicimapas is without doubt the star of my kit.
The Zumo not only speaks directions (with text-to-speech, highly amusing when pronouncing Spanish street names) but plays MP3s. The music is stored on an 8GB SD card but there is a hard limit of 1000 songs in the device, so I periodically rotate the audiobooks and music from my laptop.
I use a pair of Etymotics ER-6i earbuds with the foam earplug tips. The earplug-like isolation and sound quality are amazing, but wires are pretty frail looking. I destroyed the plug in a dirt crash on Usal Road some months ago and soldered in a replacement from Radio Shack. The drivers themselves fit nicely under a helmet.
My backup headphones are Westone UM1s, which I don't like as much as the Etymotics. They don't fit as deeply into my ear canal and the twisted-pair wires tangle too easily. Someone needs to start an "earbuds for motorcyclists" thread so we can find the right combination of isolation, wire strength, and fit under a helmet.
The extra little brown/black box between the Zumo and the headphones (clipped to the right side of the tankbag) is a Shure "Push To Hear" control
. It fits inline and has a single switch. When you flip the switch, it mutes the music and activates a microphone in the device so you can carry on easy conversation while wearing isolation earphones and a helmet. The construction of the device is extremely poor but it's incredibly useful anyways, especially when trying to communicate using a language in which I am not fluent.
One other little trick is that the Zumo 550 speaks bluetooth with my cellphone. It allows me to answer the phone and listen, but since I have no microphone plugged into the Zumo I cannot respond. I don't use this in Mexico (not at $1.50 per minute) but it has been very handy several times when riding in the US and getting calls from people that know I am on the bike.
This whole setup will get significantly more complicated when my riding buddy shows up with my Autocom. I'll post the updated diagram of my setup then.