APRS does work great, but I am not sure what coverage you could get in remote Mexico. It works on VHF a ham frequency 144.39 Mhz in the US. I think you can use it on HF bands as well for longer range, but you would need a General Class license. The VHF frequency is generally line of sight range, but there are digipeaters all over that relay the signal and transfer it to the internet. You can use expensive or inexpensive components to build your system, but it takes a ham radio license, ham radio, GPS, and some sort of TNC (Terminal Node Controller, basically a modem for your radio) Some radios by Kenwood and Icom have TNC's built into some radios (expensive), or you can buy a seperate TNC (more expensive) or use a Tiny Track (inexpensive by not many features)
http://www.byonics.com/tinytrak/. I suggest buying the book APRS, Moving Hams on the Radio and the Internet for a good introduction.
The simplest solution would be to buy a Kenwood D-700A radio with TNC built in, and a cable to your GPS. The radio can transmit your position independent of voice communication since it is dual band, dual receive. You can also connect a laptop into the system to get other station positions and know where the digipeaters are using a map with UI-View program. There are lots of options. Once the signal hits a digipeater with an internet gateway, your signal (with position, speed, heading and messaging) is on the internet at findu.com
