Am I wrong? Is the case against rationalism and therefore the case for a kind of irrationalism (or arationalism) not very simple? For the rationalist the right action, the good life, the way forward (that kind of thing) is lit up by the best argument. The good life is one based on the best possible reasons. It is surely an old observation, though, that evil can find its own reasons - reasons that can doubtless be made, given sufficient cunning, to sound equally good, if not better. If (as Kant assumed and Habermas continues to assume) the criterion has to be formal (to avoid circularity), it is likely that the reasons for evil will seem as persuasive as those for good.
Conclusion: we can describe what is good (and any mother can do that), but we cannot justify it. Anyone who asks the stupid question that was (in my experience) so often set as a topic for undergraduate discussion: "Why should we be moral/good?" is making the wrong assumption. It simply is the wrong question. A better question for philosophy classes is: Why in the hell should we think that morality is grounded in pure reason? And the question for Habermas seminars is: Why in the hell should we think that morality is grounded in discursive/communicative reason?
Second conclusion: if we are good, we are not so because we are rational. As Rousseau suspected, rationality is more likely to be a cause of evil. But in the midst of evil rational beings wreaking havoc while trying to implement the New American Century and while trying to maximise the accumulation of virtual wealth, there is obviously no point hoping that such rationalism will somehow unwind itself or run out of steam and leave the way clear for a new kind of innocence.
No, the only way forward is for a kind of rationalism that is fully aware of its own guilt - a rationalism that knows there are no clean hands, and knows that from an ethical point of view it is vacuous, and only has a content in virtue of the memory of something prior to it.