The second, much smaller, contribution of sympathetics to the pelvic viscera comes from the sacral splanchnic nerves. (Recall that a splanchnic nerve is a nerve, parasympathetic OR sympathetic, that brings innervation to viscera.) The sacral splanchnic nerves are very small, and are anteriorly directed branches from the sacral portion of the sympathetic chain. Just like the other splanchnic nerves in the thorax and abdomen, they leave the chains (on each side of the vertebral column) and course anteromedially into the inferior hypogastric plexus. These small, mostly postganglionic*, fibers are collected together, with the contribution from the respective hypogastric nerve, into the inferior hypogastric plexuses, from which postganglionic fibers are distributed to either smaller plexuses or directly to pelvic viscera.
*Sympathetic outflow originates within the thoracic and upper lumbar region. The further a fiber travels away from this region, the less likely that it would remain preganglionic. For example, thoracic splanchnic nerves, coming directly off the sympathetic trunk, are primarily preganglionic and will synapse in a prevertebral ganglion. As one proceeds to lower level splanchnics, such as lumbar or sacral, the proportion of pre- to postganglionic fibers change. For this reason lower lumbar splanchnics will have a much higher proportion of postganglionic fibers than preganglionic fibers. By the time sacral splanchnics arise, they are mostly postganglionic fibers and have already synapsed prior to reaching the inferior hypogastric plexus. In all diagrams within this module sacral splanchnics will be represented as postganglionic fibers that are coming into the inferior hypogastric plexus.
P.S. Don't forget gray rami communicantes. They travel laterally from the sacral sympathetic trunk to reach the sacral ventral primary rami.