Not hard, but I wouldn't put your new ones in the same place, there's better options, not as stealth/unnoticable though/.

I once heard that they should be mounted in the kick panels.

This was from a pro installer who really knows his stuff.

Apparently in SQ system this helps the tweeters be equi-distant from your ears.

Kinda makes sense though, if you mount them on the doors the right one is only two feet max from your head the left one is miles away.

Something to do with setting up the sound stage better

If you mount the tweeters on the kick pannels, one is still going to be significantly further to you from the other! No doubt about that. And it'll be down low with things (such as poeples feet) blocking it off. I can't see that being a good place for tweeters at all.

Where should I place my tweeters for best performance?

Okay, lets all think about staging for a second. You don't go to a concert and sit with your back towards the band right? And admit it - we'd all like to be right in front and center of the band on stage true? And you'd also like to be at the right height to - like you were on the actual stage right in front of them listening. This is the imaginary image we try and capture inside cars today when we position tweeters in vehicles. Getting this is not just a simple matter of slapping them in just anywhere either.

The problem with mounting tweeters up high (e.g. on the sail area on the door) is this:
Think about the position of your ears in relation to the tweeters. One speaker is belting the high pitch tunes out about two feet from your right ear where as the left tweeter is triple that distance away. The stage has no choice but to be right out the right window. Sheer laws of physics govern this fact. Now if you place the tweeters down in the kick panels then the right speaker distance is about three feet and the left speaker is about three and a half. The problem is not eradicated but it becomes a lot less noticeable as the distance separation is reduced.

Now obviously the tweeters cannot be placed anywhere where they fire straight into ones feet so you have to be very careful about their placement on both sides and the more often than not this results in them being mounted very high in the kick panel, quite often out of sight like mine are because they're so far up under the dash.

The next question people ask is this; isn't the stage going to be low?
The answer is not so much to do with the physical tweeter location but the power level they receive. If you have sufficient power going to each tweeter from a high quality amplifier then your tweeters will not only fill out the stage 'height' nicely but the entire front cabin of the car. That said though, serious competitors utilise ambient tweeters up high (these are much quieter than the primary tweeters however) to lift the stage a little to head height from chest height that usually exists.

And someone is bound to mention time alignment - they always do.
The biggest problem with time alignment is this; the better you make your side sound (and you can get it absolutely perfect), the more your passenger suffers. Think about it - it time delays the right side speaker so both signal paths reach your ears at the same time. The problem is that the passenger has the reverse problem to you so as you side gets closer to being the same side to side, theirs get worse and worse.

So how does one get the stage right in real world terms?
At the end of the day you just have to play around a bit (and sometimes it can take up to four hours or so) to get the tweeter placement just right to achieve a nice stage 'width' , 'height' and 'depth'. Get yourself a nice big blob of blue-tac. Stick the tweeter onto a panel somewhere and grab a song with powerful female vocals (a strong female voice is generally considered best for stage testing). Close your eyes and imagine you're at the concert. Now listen to where she is coming from. Is she singing right in front of you? Is she off to the left a tad or right? Simple move the tweeter a few inches in a direction and have another listen. How is the image? Can you hear where all the band members are exactly? How is the depth? Does the drummer sound like he is behind the other musicians? Keep doing this until you get the image dead center or just off to the left a little bit but remember to take a rest every fifteen minutes to let your ears normalize. If you attempt staging for hours your ears tend to 'hallucinate' and give false readings.

Well when you think about it. If you mount them say on the triangle bits on the doors then one is going to be roughly 50cm, the other is roughly 150cm. Down low the right one will be rougly 100cm, the left is probably only 150cm. So you have halved the gap.

That's fair enough, but the tweeters arn't worth jack crap there when you have a driver and passenger in teh front wose feet are blocking it! You wouldn't hear them much at all!

If they are amped probably you would. I'm talking about $20K SQ systems here. Not some crappy $1K system from crapfield.

a $20k system?? what a waste! but i suppose if ya got the money why not