but then fitting the new tweeter will be a bit tricky, well to get a good positioning anyway!
you could attach them to the A pliiars, have them say 3 inchace up from the dash facing each other or slightly towards the mirror - at the end of the day, if anything were to go worng, or you sell the car, they would be cheap and v easy to replace
but i woudnt advise only using the mid of the comps, as the xover points would prob be off, also the high frequencies give your brian the info of where the sound is coming from, im sure this would advesly effect the sq.
But you could try it and see, you can always put the tweets on with blue tack, thats what i, and many people do until you find the best position for them to go
OR
you could do what i did the first time
http://www.talkaudio.co.uk/photopost/data/500/3993Dash-med.jpg
you can just see, i took off the old tweeter covers, placed the pod for the tweeters in the whole (but had to remove the old tweeters) and then placed the new tweets in!
HTH
Rorrie