You can put it wherever you want as long as the wire will reach...and even then you can probably get an extension for it if you really need it. I personally also put it in the ashtray so that it can be covered up if necessary and it makes it much easier to get to. But ever since I got the iPod integration in, I haven't plugged anything into the aux jack. As far as putting it in the button bar, just make sure you are drilling into an area that doesn't have a button or any circuitry on the backside and you should be good to go.
Also, unless you are talking about engine/brakes/transmission, there's no difference between a 325 and a 330.