couldn't agree more with genixia's comments/warning, and also note that there are [from Hugos comments] - no current limiting resistors on these TTL lines (the current limiting resistors are provided by the tuner module when its plugged in) so if you don't put some in, you may your self with a dead empeg quicker than you think.

I would *STRONGLY* suggest that if you want to go down this path and use it for your TTL level GPS that you consider putting 2 Serial Transceiver chips inline (and back to back) with your GPS unit anyway - you will find this gives you plenty of protection if done right.

Be aware also that there is only +12 volts voltage available on the Tuner module interface - so you'll need a good 12V->5volts voltage regulator as well to drive your GPS and make sure tha the GPS has a common ground connection with respect to the empeg or you may find yourself killing your empeg through ground loops and voltage spikes...