I'm considering upgrading from the AVIC Z3 to the Z110BT (might even buy 2). My question is about the wire harness. Are they easily interchangeable, or will I have to install a new harness?

You will have to install the new harnesses.

grrr, ok then. Well, at least everything else is already installed. Should be able to use the same GPS receiver and vehicle sensing wire I hope.

Not sure what the Z3 GPS plug looks like but if they are different then it should be a simple swap of the plug adapter. I know from D3 to z110bt thats what I did.