bit late on the response but it's a different situation. Pleading the fifth isn't the same as not knowing or forgetting something. Murderers don't bury bodies then when caught say that they know where it's buried but aren't telling. Even if this was the case and they gave the defendant immunity for knowing where the body is, once they found the body they could use the body to try and find evidence to link the defendant to the murder. It's just like saying "ok well just because you know where the body is, it doesn't mean you murdered the person".