You ask, isn’t it now no longer possible to ask the [Previous] Rebbe, my father-in-law, of blessed memory, when one is in doubt which course of behavior is correct? [This is not so, for] if you stand firm in your Hiskashrus [bond] with him, and pay no attention to the lures of the evil inclination, and you send the request to the gravesite of the [Previous] Rebbe, my father-in-law, of blessed memory, the [Previous] Rebbe will find a way to answer you.In my own words with some explanation: The clear lesson from this for us is that we should not think that since we do not see the Rebbe after Gimmel Tammuz, it is no longer possible to write to him, receive his blessings, for him to davven for us, and for him to answer our questions concerning appropriate behavior.
Igros Kodesh, Vol. 3, p. 266.
Thus, when one writes a letter to the Rebbe asking for advice and sends it to him, the Rebbe receives the request, listens, davvens, and “will find a way to answer” just as much as before Gimmel Tammuz.
What does it mean that “the Rebbe will find a way to answer”? It apparently implies that some event will occur, or the person will reach a realization, or the like, that will answer his question, and that he will sense that this answer is indeed from the Rebbe.
However, for this approach to be effective, one needs to maintain a strong Hiskashrus. The reason for this appears to be twofold: firm Hiskashrus makes one worthy of the Rebbe’s communication, and sensitive to it.