Agree with SMAN and fredberg. The triangle cover is a good location. Should be a very easy install, you keep all the wiring inside the door, you don't have to cut a hole in your door panel, and the tweeters are mounted higher than they would be in the door panel.
I have a 95 XLE with the tweeters in the door handles. When I bought the car they were fried so all my high end was coming from the lower door location. I recently replaced them with a pair of Polk Audio DB1000 tweets and it REALLY improved the high end. I chose to stick with the factory location, otherwise I would have mounted on the triangle cover for all the reasons noted above.
Stereo sound is very subjective/personal, so I'm not questioning MrSo0h0o's experience, but I don't find the sound to be harsh at all in the XLE door handle location. My Sequoia has the tweeters mounted up in the corner of the windows and I don't find that to be harsh either.
When you remove that triangle cover, be careful/gentle. It's relatively easy to crack it.