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You want to pick surface tweeters that mount easily to your pillars with minimal damage to the car. The HU internal amp is only 15 wrms/ch, so pick efficient tweeters. Go to your favorite store and listen to some. Put them in parallel with the rear Bose Twiddlers wires which are available on the back of the HU.
High cost does not always mean superior sound. Lower cost tweeters tend to be more efficient and should sound OK on 15 wrms. Go to the store and listen to something you like. Any tweeter will only add to the system. A $20 set of tweeters with a simple capacitor crossover will do the job. If you don't like them, you haven't lost much.
Also pick a surface mount design that you can attach to the pillar or dash with minimal damage. You can even use an adhesive that could be removed later.
99HT is absolutly right, I had a $99 set of Kenwoods mounted on the pillars in my 77 camaro w/ a cap on each one and they sounded great.
then in my 88 z28 i had a set of $20 dollar audiovox w/ a cap on each and there was little difference, Yes I did like the Kenwoods better (Much clearer on things like cymbals and junk) but for the price difference the audiovox's where very good.