Positive Ground Battery Connection

[72Thunder]

Hi all,
I have just started the tear down process of my '72 C10 and when it came time to take off my passenger side fender I noticed my positive battery wire had been grounded to which looks like a factory piece on the fender. We thought it was alright cause the truck could start and drive just fine but as soon as we figured we didn't need it we took it off it but it never powered up. So we found a new "ground" for it (rad support), it started to spark and eventually caught on fire. i am confused by this as after everything cleared up we put it back on the fender and everything started up all fine and dandy. Is there something I'm missing?