If you do a search there is a lot of info on this. But to answer your question:
To replace a tweeter you just have to wire/solder a new tweeter in, but you also have to figure out how to mount it, probably with glue, it's the same with the 4" dash speakers.
If you search there are a lot of instruction on taking the door apart to replace the woofers (wikipedia), and I am not sure about figuring out which speakers are busted. I would try placing my hand over the speaker grills with the volume up to see if you can feel the air movement. And before you go dissembling the door just pull the dash speakers out and look for damage. The dash speakers are a lot easier to replace than the woofers.
Also, if the factory system is alright with you, you can buy new dash grills with the speaker and tweeter already attached.
Hope this Helps!
Porscha
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