I flush mounted Vifa Neo Tweets in the doors directly above where it says "stereo" (right by the mid). They look factory and sound very good. However, it took me 4 tries to find the right tweet with a good off-axis response to sound good mounted like this in that location. Tweets that can swivel to aim them towards your ears would also work well here.
By all means, avoid putting tweets in the B-pillars.
