You can use these with any deck, stock or aftermarket. But the lower the wattage output on the deck, the crappier they'll sound. If you can afford it, I always recommend going with an amp. Clearer sound (even at lower volume levels) & no need to crank the volume knob as high. All around a much better sound than unamplified.... But will they work without one? Yes.
I'm not too sure what you mean. All cars and trucks have a unique wire harness that plugs into the factory speaker's similar plugin harness. These Pioneers speakers only have two basic male pigtails. Which means that either you have to cut your wire harness in your car and add female pig tales, which is very messy, or you need to do a search for the proper adapter that adapts from your harness to these pioneer speakers. Copy this: << Metra 72-9300 Speaker Connector for Select Mazda/Nissan Vehicles >> Look at the product. Now, look that the big plastic end. Does that closely match your car's speaker wire harness? If it does, then this adapter is what you need!
That depends on what your amp output is. If your amp only supports 2 ohm speakers then no they will not sound good these are 4 ohm speakers. I got a pioneer deck and a amp bypass wire and they sound awesome!