Yes, I know, I thought the question was why Harry didn't send him a card.
Easy misunderstanding of what I was addressing because there's a lot of other points raised by others between our back and forth, so I've collected ours right here for ease of reference:
Yes, communication or lack there of, goodness knows, has gotten Harry in trouble with friends etc before, but not this time. Carlos did communicate quite clearly with that tracking dot, he was telling Harry, "you're a suspect.." When he says, "talk to me.." He is saying, "confess.."
I agree with this as far as PT is concerned, but think Harry could have salvaged the relationship by communicating more before PT. It was risky and maybe the right call at the time not to, but it could have worked.
Should ah, could ah... But Carlos was apparently in the hospital, Harry was busy with Maggie and Murphy, not a good climate to talk.
He's known Carlos for years. WN or right after was the right time.
Now to where we are:
Which he made clear after he found the tracking dot on his wrist.
So in the context of going back as far as WN, I don't think the dot on his wrist means that Harry's trust would have necessarily been misplaced in Carlos. I do admit the dot is some evidence that it might have been misplaced. On the other hand, a lot has happened that hadn't and might not have by the time Carlos puts the dot on Harry's wrist. Harry sharing secrets with Carlos could have built more trust so that Carlos would have been open about his concerns instead of sneaky.
I think Harry did tell him a lot, pre Changes.
I don't think he told him anything big. He didn't tell him about Thomas, Elaine, Lash, and probably not Uriel or Luccio either. Telling him about Lash, Uriel, Thomas, Margaret the Elder being their mother, Lord Raith's murder of her, her death curse, and Lara's détente with Harry could have settled a great deal of suspicion Carlos had for Harry's actions and knowledge. But again, it also could have resulted in anything from pressure tactics against Harry to his execution (both from the White Council).
Knowing what we know of Harry, I would trust his intentions if not necessarily his judgment.
However it is "post" Changes that has Harry where he is now, and from the books at least I don't think they had much if any contact.
I agree that post deal with Queen of the evil faeries is probably too late to build that trust. Maybe not, but I wouldn't bet on it. For the sake of discussion of what Harry should have done, where he is now isn't all that relevant. Where he is now is thrown out of the Council and not friends with Carlos anymore. That's relevant to what he should do, not what he should have done.
I'm not sure if it would be a good idea, but I wanted Harry to tell Carlos that Thomas was his brother at the memorial service in BG. It basically explains all of his actions in regards to the White Court. The reason stated by Harry that he kept it a secret was that the WC and the WC would both try to use it against them if it was known. Harry only has ties to one of those WCs now. That reason is now gone.