I am not trying to make any one sound stupid but is the battery charged?

 

if its an automatic,it could be the neutral safety switch

 

I'm not sure about your problem, however, I found that cleaning, meaning file/sanding, whatever, of all, repeat all the grounding straps on your vehicle you might be surprised how well things begin to work.
Been there done that!

 

Your truck may have a fusible link by the battery which is a small wire in between two larger wires but I'm not sure if a 85 has one or not