Where are interneurons in Autonomic Nervous System?
It sort of depends on your definition of interneuron. Many school-level textbooks suggest that any neuron that has neurons both pre and post synaptically are interneurons. Therefore both the preganglionic neuron and the spinal sympathetic interneurons could be considered interneurons. This means that all neurons in the brain are interneurons and vastly oversimplifies the CNS.

I would argue that it's probably more helpful to think of the preganglionic neurons as "projecting" neurons, as they come down the spinal cord. The post-ganglionic neuron is more like a motor neuron.

Edit: I answered a similar question in more detail here: Is the bipolar neuron of the retina considered a sensory neuron?