I'm willing to bet the first two channels have the higher crossover points. But to be absolutely sure look at the amp crossover to see which channels need what speakers. Typical crossover on amp has given cutoff points for each set of outputs.
So depending on which amp you have it, the tweeters usually are the first two channels. Mids will be channel 3 & 4 obviously. If you have an active deck the you can use any configuration you want.
I just finished setting up the 12 channel active front stage in my blazer. Worth every bit of the time i put into it.