In Kant's original argument he was not trying to show that god exists, oh no! instead Kant was trying to show that if your going to be moral AND rational you must also believe in god, or in other words, to be moral and an atheists is irrational.
so Kant's (roughly translated) original argument goes:
"The highest good is where moral virtue and happiness coincide, and morality is the pursuit of this highest good. In the world we inhabit it's not possible for this highest good to exist, because wicked people prosper and good people suffer, etc. If it's not possible for the highest good to exist, then it's not rational to pursue it - ie, morality is irrational. Therefore, in order for morality to be rational, we must believe in god"
Kant is basically saying that while god may not exist, we must act as if he does and that he will punish us for failure to act properly because only with this belief can we act morally and still be rational.
Note the direction of the argument's thrust. Notice it is not god Kant attempts to prove, nor that atheists are irrational, nor even that god is required for morality (an implied premise), instead he attempts to argue that if we wish to be moral and be rationaly consistent we must strive to believe in god as well. His reasoning is that unless we have punishment for our actions hanging over our heads the only rational choices are amoral ones, which leaves us either moral and irrational or amoral and rational. A devilish (false) dichotomy.
The problem is that it's easy enough to show that people are moral without god, this leads to one of two conclusions. Either people can be moral and rational when there is no god, or the vast majority of people will act morally even over rationality. As the second conclusion seems very unlikely (as rationality and morality has tended to go hand in hand), the first conclusions seems to be correct which means there is a problem in Kant's argument. It's easy to see where as well.
My behavior modifies the behaviors of others. if I wish the behaviors of others to be good towards me then I must act in such a way to modify their behavior to my benefit. We call this moral behavior. It is purely rational to act in such a way to cause others to act in a way I like. The 'moral behavior' to be used depends on the context of the society I live in but this doesn't hurt the argument, in fact it enhances it.
The argument misses intent, the intent of moral behavior is not to make your life a wonder and get everything you want...that is simply a fantasy that very few can achieve (even if they take the route of pure selfishness and attempt to lie and cheat and steal), SOME can...but by far the majority can not. Instead the majority of people take the rational moral route because that is the only route that will afford them gaining some of what they want...namely to be treated decently within the society.
Kant's mistake is that he attempts to disconnect my actions from your behavior. My actions modify your behavior. If you ignore this factor then of course not lying and not cheating and not stealing and the rest of morality will not work and the rational choice is to cheat and lie and steal...because as the argument is presented it makes sense to get what you can. With the factor of society it's easy to see how the entire interconnection works.
Think of it as comparing a child's morality to an adults.
A child needs to know that his actions have consequences. Those consequences are presented through an authority figure. The reason is simple. A child can't understand or even conceive of the complex interactions of give and take of moral behavior in a society. The child doesn't understand that being bad leads to bad results because others will be more likely to do the same to him...no the child needs to know there is an authority figure that will hand down justice.
An adult has a more developed morality and does not need the authority figure to met out justice for crimes or proper behavior. Unfortunately because of the childhood past the mental authority figure model of morality is deeply entrenched mentally but only because that is the only formally presented moral structure. When someone talks about morality this is how a person actually thinks about it. But when we are forced to make a moral decision we rely more on our day to day experience. The schoolyard interactions. The daily 'tit for tat' of work and school. I'm good to others because they will be good to me if I am. If I am unfair to them they will be unfair to me (when they can).
Kant's argument devolves to more steps but it is still the same:
I can't see how you can understand morality in any way but with a moral figure, people are rational, it is irrational to be moral if there is no moral figure to enforce justice (or at least it's irrational to be moral if you do not BELIEVE in such a moral figure), therefore god exists (or at least it is rational to believe god exists).
Other parts to this series:
Arguments for god...kind of (Part 2) - The Kantian Moral Argument.
Arguments for god...kind of (Part 1) - Pascal's Wager.