you need what is called either a line out converter or line level converter (they are the same thing). walmart should have a cheap quality version for ike $30. but i'd suguest buying online for a better quality LOC. partexpress.com has this to offer:
yeah you just connect the wires to the ground of the deck and the rear speakers install done. try to get on with adjustments to tune out voices and other noises. The ones at wallmart don't have that function. If you want a step by step just PM me.
it depends on what amp, not all of them are high quality, they are getting better but not as goood as a david navone LOC, the david navones ones have adjustable boosts and gain, no mattter what you'll still have the use of the adjustable crossover. the bass boost on the david navone loc's are alot better than the adjustable bass boosts on the amp or hu, they won't distort it as much.