Cheap electronics is typically not very pretty inside and this one is surely no exception. It is a depressing fact that volume products like this are made by hand...

Here, I decided to keep the LED as well and therefore added a separate resistor for the opto-coupler. The simplest way is otherwise to cut the wires to the LED and solder them to the LED inputs of the opto-coupler instead. The red and black wires are connected to the red led on the chime.