About the speaker Design
u can directly install the new speakers in the door. however there will be 2 problems. 1, u will have to drill up some new holes for the speakers. 2, once installed, chances are ur window will most probably hit it every time u roll it down.
so u will have to make new spacers to the same spec as the plastic ones with the 3-hole design. the new speakers will mount onto the new wooden spacers.
About the Aftermarket Speakers
3k wont get u the best coaxials out there but the ones u can get will definitly sound alot better then the stock ones. u already told me u were getting those jbl's. visit the shop i told u about and see if they have the same model there. else u can find a lot of options in sony xplods and pioneer
About the Sound Quality
changing the speakers wont give u the best sound. try sound deadening ur doors. see the holes in the doors. fill them up with either jambolen foam, rubber layer or proper sound deadeners
u can then put back the plastic sheet on top of these.
also cut the foam in a shape exactly to the specs of the new spacers and put these behind the spacers. this is done to seal it off and not let any air out of the sides.
Hope this helps
