The battery is not grounded to boat ground. The mast-to-keel bolt ground is I believe what you are calling "boat ground". As Petobysmol stated, the mast is grounded for lightning protection to shunt the strike to water through that ground cable (usually pretty heavy guage) as opposed to the strike finding whatever path(s) it wants to in order to get to "earth" (the water).
Once lightning strikes, these "path(s)" can travel anywhere through your boat. I was lucky about 3 years ago with a lightning strike that ONLY vaporized my VHF antenna, blew apart my wind indicator, fried my VHF and bilge pumps, and finally blew out a few fuses and sockets. I never really fully understood the pathS that my strike took, but it could have been worse...blowouts through the hull from the strike travelling down the shrouds to the chainplates and through the hull are not unheard of.
The lightning strike takes the path(s) of least resistance to 'earth' (the water)......so give it one, straight to the keel bolts and the water (where it wants to go anyways), and check your connections ensuring they are tight and in good condition no less than yearly. It's much better that YOU control the strike's path(s), as best you can.
Sorry, I diverged... you need to 'hand over hand' your wiring for breaks (opens) and/or corroded connections (high resistance connections)...all terminations / connections MUST be metal to metal. Also, if it's the original wiring, I suspect that back in 1985, they MAY NOT have used marine grade wire (tinned copper), and used the standard automotive wiring (copper only). Automotive wiring is NOT made for a corrosive environment and after time, will oxidize turning a black / dark brown color, which 1) makes it brittle, and 2)does not conduct electricity nearly as well. Thus susceptible to 1) failures and 2) low light output at your light fixture (low amperage).
Once you've checked / repaired any connections a/o replaced with marine grade wiring, it's a good idea to 'insulate' the terminals / connections with, 1)heat shrink tubing, 2) electrical tape, 3) lightly spray with WD-40, OR 4) enclose the connection in a waterPROOF box and seal off the wire pennetrations with silicone caulking... just a few ideas.
Good luck, electrical stuff is always "FUN".