i have an impala, there is a tweeter right next to the 6.5 inch speaker, when i put the new 6.5 in, i didnt really have much room for a new tweeter. so i put my tweeters in the air vents where the heat and a/c comes out. i have heard nothing but good things about this spot, they sound so much beter there then they did in the door, and no one even knows they are there
Either double sided tape, or consider a braxial style mount (using a metal bracket that holds the tweeter on top of the main speaker, similar to a coaxial.) CDT makes braxials if you want to see what I'm talking about.
the first thing i cheeked a million times was to make sure the air got though, i live in houston and it can get HOT, no air was blocked. i used a a metal bracket and bent it, a 90 degree angle, screwed the bracket in first then the tweeter on the bracket. you dont need much bracket about about 5 holes worth.
well with my car you have to pop off the front datch, and when you do the vents are right there. i guess they would be considered inside ones, the only way to get to them and see inside them is to get the datch off. if you are going to go this route you need to play with your vents and decide how you can get to them.