Final answer:
The RT should position the light source over the right upper chest.
Step-by-step explanation:
The RT should position the light source over the right upper chest to assist with the transillumination of the neonate's chest. Transillumination is the process of shining a light through body tissues to assess the presence of abnormalities. In this case, the RT is assessing the chest of a neonate who is intubated and receiving mechanical ventilation (MV). By positioning the light source over the right upper chest, the RT can illuminate the area and observe any abnormalities or changes in tissue density.