I have an '84 4runner with some rust around the rear taillights and was considering cleaning up the whole underbody and then sealing with the undercoating material but...there was no rust. Now, for all I know the only reason the rust was appearing near the taillight assembly was maybe because they got scuffed up against some trees and just began rusting from there. As for the underbody, all that accumulation of grease and gunk, oil from the road, etc, etc...is probably PROTECTING the underbody from rusting, you know. I believe the best thing I could do is...leave it alone...unless there is indication that rust is developing there.