You could just go to pin 2 without a relay but then you'd still have constant power and would have to remember to turn on and off the garmin. Pin 7 is Run/Crank feed at 10 amps.
If you don't want to deal with it I sell a pre-made plug-and-play harness with a cigarette plug for $34.99 shipped that take 30 seconds to install. PM me if you'd like more details.
For those of you that are looking for the connector I put together a kit with the connector, lock and pins:
http://www.polymotoengineering.com/GMT900CK.html
Please excuse the website, still working on it
That's the right one. Pop the red clip out and use a really tiny screw driver to disengage the tap that holds in the pin/blue wire. Now insert the pins as shown above except don't cover them with shrink wrap and reinsert the red clip.
I could never get mine to work and after A LOT of searching I found a service bulletin for certain types of valve stem type (rubber I think it was) letting air out causes a harmonic vibration so the sensor thinks it rolling instead of sending position information and that the programming tool was the only was to get them to register.