I'm not a radar tech person, but I would assume (hope)that the radar has a 100% efficient reflector to make the radar waves transmit forward. If for some reason the reflector is not 100% efficient, some radar waves may move backwards (or any other direction). Now, if the cabin pressure bulkhead behind the radar does not stop radar penetration, then some radar waves may enter the cabin. Clearly, the further back you are, the less exposure.

This is more an academic statement and I have no proof so would welcome clarification.