What is the test for appointing a substitute attorney at a trial?

"[A] trial court must conduct [] a Marsden hearing only when there is at least some clear indication by the defendant, either personally or through counsel, that the defendant wants a substitute attorney. . . . [I]f a defendant requests substitute counsel and makes a showing during a Marsden hearing that the right to counsel has been substantially impaired, substitute counsel must be appointed as attorney of record for all purposes." (People v. Sanchez (2011) 53 Cal.4th 80, 84.)