I have had two 1972's that had body filler in that place and the sail panel. Both had been re painted and may have had the lead replaced as body filler will not stick to lead and body shops refuse to deal with it. I have leaded both areas and welded the lower corner where the sail panel meets the rear window. These were a problem with both of the cars. I am hoping that with my frame strengthening efforts and lead filling this will curb this problem. I have seen the cracks appear in cars with as little as six month's after their re spraying with body filler only.