I dunno about installing physically, but electronically, I guess the easiest way would be to get some kind of multi-pole swith that you can fit in where the ISO adapter is. When ever i've installed stereos in cars i've left the stock wiring intact and cut throuh the replaceable ISO adapter so that if i need to return to stock I can.
The most effective way would be a bank of relays that you can switch from the dash that would just flcik and that changes the position of all the relays.
If you do it further along the chain you'll have to remeber to do RCAs and amp remote, but also route all the incoming leads too (lights dimmer, constant power, switched power, power aerial, aerial in etc etc). doing it all around the ISO adapter means you have it all in one place and you got nice colour coded cables!
It'll be a bloody pain in the ass tho wiring all that up.