With a vehicle with MOST, it is not really that difficult to swap out as you may think. Lots of the 9X7 guys and 9X6 guys have done it before.
The difference in cost between a hu replacement in a non MOST and MOST vehicle should literally be the cost of a new amp & corresponding wires plus a little more work for the amp mounting/connection. As for the speaker wires, you simply tap the signals at the speaker wire harness currently attached to the factory amp.
Here is a link I found that has color identifications for the speaker wires that may help you when you want to make the connections (see post #7):
http://www.6speedonline.com/forums/996/226668-pioneer-z120bt-about-install.html