One thing I have found is when I have the Garmin hooked up to the 12v plug, when I go to start the bike the power draw from cranking it restarts the GPS and then it has to grab satellites again, very annoying. I would like to figure out why, because plugged in its nice because the back light stays on all the time. Any ideas guys?

 

It probably interrupts the 12v connection when the starter is engaged. Mine has a warning if external power supply is lost.....

 

Sounds like you're getting a voltage drop to cause the GPS to reset itself.

Deej, you have the wiring straight back to the battery right? Via a relay?
If its via a relay, is the relay dropping out?

I don't know there is any disconnection of any of the bikes electrics during cranking like there usually is with say a car.

If there is no designed disconnection, you have wired your socket straight to the battery, there are no loose or poor connections and you have used thick enough wire (shouldn't be an issue powering just a GPS) you could try a largish capacitor wired in parallel to the socket and wired as close as possible to the socket. This will help remove the sudden drop in B+ when the starter is pushed. The value of the capacitor used will determine just how affective this is.

My set up doesn't do that but the Zumo has an inbuilt lithium ion battery and any interruption to the 12v supply causes a "do you want to power down" warning. But this not happen on cranking.

Has it always done it? Possible battery getting old?
If the voltage drop is evident across the battery, and your GPS is sensitive to this, there won't be a lot you can do. Try a capacitor. Usually quite affective in storing and delivering quickly enough energy to the devise to compensate for the B+ drop.