We can trust the science of the brain or science of the {anything else} because it gives us predictive power for matters in the universe that, when repeated numerous times, can be reliably said to tell us something true about how the universe works. All scientific truths are provisional, but when repeated beyond all shadow of statistical chance, can be treated as facts.
Besides, I dislike your whole tact here. If brains can't produce trustworthy thoughts, how do you believe anything with any degree of certainty? I'm guessing your very worldview contradicts this idea, simply because you have a worldview. And If you want to continue down your path into full-on subjectivism, fine, there's not an actual refutation for that except to say that it affects religious ideas equally as much as scientific ones. But it doesn't really accomplish much from a debating perspective.
There's also the matter of neither you nor I properly understanding just how numbingly complex brains are. But I don't see how it's a stretch to think that we could understand some things about the universe around us when we ourselves are a part of it and made of the same stuff. I think you're just going to far. Are our ideas infallible? Of course not. Subject to error? Of course. But not able to be trusted? That's a giant leap from the first two questions. Individuals thoughts, perhaps, but collective and repeated thoughts, put to tests to confirm or deny them, those can be trusted.