the speakers didnt affect your trucks navi b/c there was no electric pulse going through your speakers. once you get your speakers hooked up and running, the electricity is what makes the magnets move and pulse, which is how it reproduces sound.

ive never heard of this problem but im sure its possible that the aftermarket speakers could jack up your navigation. as far as insulating the magnet area, im not sure if thats possible, but it sounds like a good idea. those kickers must be pretty high in power, haha.

anyway, sorry about your situation and i hope someone else can give you more help. and congrats on your new car.