A shopping list from digikey. Skip the first capacitor- it is too small. Actually skip them both and get a car audio capacitor from Wal-Mart.
Most anything should work. 0.5-5F (Farad) would be fine. The 100,000 uF cap from digikey is a 0.1Farad cap. The Walmart ones are ~10x the size for the same $25.
The (big, metal can) resistor is to limit current when you reconnect the battery (or else BANG). You want it low resistance, like 0.33 ohm so the GPS doesn't lose voltage thru the resistor. But enough that when you connect the cap (or turn the switch on the house battery), the rush of current into the capacitor doesn't blow it up, or the fuse. It is a balancing act. You might need a 10A fuse to the cap, or a 15. And then a 3A fuse after the cap(acitor) inline to the GPS.
The diode keeps the capacitor from feeding anything but the GPS unit. Which limits you to a 10A fuse. The diode is the check valve for electricity. The silver band is the output (GPS) side. You will have to do some soldering.
Equivalent parts might be available from digikey.
Schematic to follow.
I am figuring your GPS doesn't draw more than 24 Watts. I think mine uses 8 or 9?
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