Wow, the distance from the lower part of your door to lower A-pillar is about what, 18"? That would necessitate a crossover point no higher than 750hz to eliminate lobing.
Your tweeters will not be able to handle that low of a XO point.
Without knowing anything about your tweeters, you probably better not cross them any lower than 2.5khz. You can try lower, you can try higher, it's up to you.
Here's a few passive XO's to look at (stick with the one's that are around $23/ea).
http://www.parts-express.com/w...t=193