Well, consider this. Does a RentalEquipment contain a CustomerOrder, or does a CustomerOrder contain a RentalEquipment?
My guess is that it's the latter, unless you have a very odd system.
Given that, a CustomerOrder should contain a reference to the RentalEquipment it contains. You can have a status flag on the CustomerOrder indicating whether the order is still open, or the RentalEquipment has been returned.
This also makes sense if you want to track the order history of a given RentalEquipment.
If it's a foreign key in CustomerOrder, you can look through your CustomerOrders to find the history. If you put the relationship in RentalEquipment, you'll only have information about the CurrentOrder. Also, when the customer returns it, you'll have to update the RentalEquipment record and set the FK to null. Messy.