Jason Seremwe

Hi, I drive an old Ford Transit Van 1999 diesel engine and recently it developed smelling strongly a bad smell like burning sulphur or burning electric cable especially when i am cruising on the highway. The smell is even more if i try to open the window and it seem like it fills the back of the cabin more that the from seat. I have tried change my battery to a new one , my front and back crawl seals but the smell has not left. What could be the problem? Thank