Back of cab rust was caused by ??? But, here is a picture of the bed showing the rust next to the cab. Several inches of the front of the bed are completely rusted away. Probably from leaves stuck between the cab and bed as suggested. The truck sat for 25 years in a swampy area next to an old building while a tree grew over it. Nothing but rust under the bed. Sills are mostly rusted out, especially front and rear pieces..... thanks for the help
