I don't have the answer to your question, but what would give you the answer you seek is a picture of the backside of a rear cap. Maybe the factory could provide you a picture and some advise. blockages might vary pending model. Assuming you have a clean vertical channel in the rear cap, I imagine you'd have to treat the project like at home working in between walls. You'd have to drill a hole where you want the light, fish a wire from there down to the floor where you'd be drilling another hole to get to the underside of the coach where the tail lights are.
First call Kermit Fisher there at the factory. He is not a lover of email. Best to discuss over the phone.