First let me say that the circuit described by Toyota will work as long as the device does not require good voltage regulation. Most of the GPS 's that run off 12 volts like the Magellan Sporttrak Topo have a wide input voltage spec. Just be sure to include the electrolytic cap on the DC side of the bridge rectifier because if you don't you will get some very high spikes (the open voltage output of the magneto is >50 volts). The risk with a circuit like this is that there is no protection included. If the AC regulator or a diode fails you will likely fry your GPS as the DC voltage will climb to the full output voltage of the magneto or maybe just send AC to your GPS. Either case would probably not be good for the GPS. If you decide to go this route at least put a fuse (maybe 1/2 amp) in the lead (yellow wire) coming from the magneto. The Etrex is a different animal as it is spec'd at 3.0 volts +/- .5 volts. Using a resistor in line does not provide a constant voltage to the GPS. It will vary as conditions change (i.e. - input voltage, current load, etc) and I would not recommend it. A better solution is to use a voltage regulator like an LM317 ($1.99 at Radio Shack) and a few resistors/capacitors to build a regulated power supply. It is a very simple circuit (not much more involved than the one Toyota described earlier) that will provide a stable source of regulated 3 VDC. I don't have any means of posting a picture but if you PM me I'll draw up a schematic and email it to you. I'm not trying to rain on the parade here, I'm just trying to make sure everyone is aware of the potential problems before they risk they're prized electronics. BTW - I'm a retired engineer.