Run a 3.5mm Stereo extension cable from the audio output port (on the wiring harness) to the Auxiliary Input on the front of the radio (or install an additional Aux input port on the back of the radio)
If you plug the GPS into the Auxiliary Input don't you disable the radio an IPOD sound from coming through the speakers.?
If you run the GPS outputs to the AUX input on the radio, yes, you lose all other inputs on the radio IF you want the GPS to talk to you through the speakers. In this particular case, there would be no advantage in buying the Roadtech 665 over the regular Zumo 665. Someone correct me if I'm wrong, but the main advantage of the Roadtech (HD) version is that it will allow full functionality of the radio while still talking to you for directions IF you also install the NIM (Navigation Interface Module) that HD makes. The NIM wiring harness is not stock on the SESG, so that would have to be added as well...that is the reason they (HD) say the RT 660/665 will not work on the SESG. But it can be done, and has been done by some members here. The only thing you CAN'T do with the RT version, NIM, and wiring harness is listen to MP3 tunes loaded onto the Roadtech 660/665...if you want to do that, you still have to have the stereo patch cord from the unit to the AUX input, but who GAS about the MP3 capabilites of the RT when you've got XM, Ipod, FM, MP3 CD, and AM on the radio? I'm also assuming that the SESG has the XM module already on the radio? If so, the only extra advantage of the Roadtech 665 over the 660 is the weather radar function which requires the Garmin XM antenna module. So, if the SESG already has the HD XM radio module and antenna, the RT 665 would only be using the Garmin XM function for Radar, and if for some reason one wanted to have that XM inputting to the radio, which would still require the use of the 3.5mm stereo cable to the AUX input on the radio, and you're back to square one.
There are folks on here who know a lot more about the electronic functions on the HK radio and how it all interfaces with both regular Garmin products, and the Roadtech version of the same product, than I do, so someone please correct me if I've misinterpreted anything in the sequence of operation stated above. But either way, it can be done. It all depends on how important a "seamless" audible GPS direction is to you.