Stock head unit has built in crossover points to make factory speakers sound good. You need something to flatten the signal if you want your aftermarket speakers to perform adequately. I started a thread about my custom stereo and included a parts list along with a youtube video about it. Search my username to find it.
Adapter plates can be used for 6.5" and 6x9's in the doors. I also used adapter plates to mount 1" tweeters in the dash. Factory speakers are carp. Any questions just let me know.