Relative afferent pupillary defect (RAPID) is characteristically seen in damage to -
Relative afferent pupillary defect (Marcus Gunn pupil) can be seen in lesion anywhere along the pathway of afferent pupillary pathway, i.e. retina, optic nerve, optic chiasma, optic tract, or mid-brain (pretectal nucleus). However, RAID is most characteristically associated with optic nerve.
This thing I will have to explain in detail.
Two types of lesion can occur in afferent pupillary pathway :-

i) Total (complete) afferent papillary defect :- This will result in amaurotic light reflex (amaurotic pupil) i.e. absence of ipsilateral direct and contralateral consensual light reflex. However, it should be kept in mind that complete amaurotic pupil will occur in lesion of retina and optic nerve. In lesion of optic tract there will be Wernicke's hemianopic pupil because fibres from nasal half of retina deccusate at optic chiasma and go to contralateral side. In other words (according to me), hemianopic pupil is nothing else but amaurotic light reflex but the only difference is that the light reflex is absent when light is shone to temporal half of ipsilateral retina and on nasal half of contralateral retina. While in true amaurotic light reflex light reflex is absent when light is shone to the ipsilateral retina whether temporal half or nasal half.
ii) Relative (incomplete) afferent pupillary defect :- In relative afferent pupillary defect there is Marcus Gunn pupil.
However (as in above discussion) classic Marcus Gunn pupil (RAPD) occurs if there is incomplete lesion of either in retina or in optic nerve. In incomplete lesion of optic tract Marcus Gunn pupil like reaction will occur when the light is shone to temporal half of ipsilateral retina and nasal half of contralateral retina. However, it is not classical RAPD which is seen in incomplete lesion of optic nerve or retina.
Optic nerve
Optic tract
Lateral geniculate body
Occulomotor nerve