I installed a new set of speakers the other day. I chose Hertz DSK170.3 6.5 speakers. Woofers fit without any modifications and they have to correct hole pattern for easy installation.
The tweeters needed a bit of work. I cut some tabs away from the original speaker surrounds with a knife. Then I could just push the new tweeters in and hot glue the whole thing together.
The new speakers came with a crossover box. I taped some cell foam around the crossovers and tucked them into the doors thru the woofer speaker openings.
You propably could use the original speaker wires. I run new cables for the tweeters because I could not find a way to split the old cable without taking the whole dash apart. Basically I connected the original speaker cable to the crossover Input and then connected the new speakers to the crossover outputs.
Then I crimped new terminals. The old ones do not fit so you'd have to replace them anyways.
It took a couple of hours to do the whole thing. I replaced the head unit while I was at it. My car had a very ugly looking AEG radio. Now I have a slightly better looking Pioneer.
Didn't need a hammer. Screwdriver, knife, hotglue gun and a crimping tool was all I needed.