2007 E350 Mouse Smell in Heater

When I turned on the heater this morning there is the distinct mouse deposit
smell.
It is fine when the blend is on cold so AC is not affected
It comes in when blend is on heat.

I am assumung the droppings are in the heter core box or the ducting before
the blend door.

I am not familiar (yet) with the ducting system.. looking for advice on where
to start looking in some sort of logical manner

Thanks