totally depends on the vehicle and the install. you don't want it where it can get wet - so not in the door. on the door is possible if both mid and tweeter are mounted on the door. it just depends on the available space with the door panel. I've also located them in the door pocket.
I've placed them in the glove box - if they have a lot of settings to access.
I like them back by amps for wiring convenience, but you need twice as much speaker wiring.
You won't hear a difference if they are close to the speakers or at the amp. Just find a location that is safe and dry.