He settled in the Impala an let the silence drag out for a bit. His eyes were focused out the window and part of him didn't want to bring this up seeing as how Dean seemed to be in a good mood right now and he didn't want to be bothered with the argument, but the more he let it drag out, the less time he'd have to convince Dean of it.

"Dean, I was thinking... Maybe it's not enough to smash just that mirror. Mary's hard to pin down, she can move from mirror to mirror obviously. So... maybe we need to summon her, tie her down,and then smash it."