I hope its something simple like this.
I had this problem on a different Vehicle 1960VW bug.
I did all sorts of things and finally I decided to replace that connecter to the starter solenoid, the wire was bent and unraveled.
I cut the old connector off and replaced it and the sun of a gun fired right up.
I saw it when I first worked on it but kept ignoring it because it was connected. The person who originally installed the wire connector stripped the wire two long for the connector exposing to much wire allowing it to come unraveled and letting it get corroded.
Do you have good battery cables?
Good battery Ground strap? Dissconect the strap and wire brush the contact area on the fire wall.
I always wire brush all my connections on a project, fuse box, straps, cables,terminals, replace fuses, Just to eliminate any goofy simple problems. It easier than pulling a starter.
