I would unplug / undo the Alternator & startor motor main power wires & see how you go from there, i pressume the battery post is where the fused main feeds for the ignition etc is taken off from ? & also take off / undo the fused main feeds / wire to the car electrics from the battery post to, but do it all in stages so you can see what hopefully works..
I pressume you have triple checked both batteries are being connected correctly, also take the old battery out because if its really dead flat they sometimes draw quite a current initially when you connect up a good battery to them, which will cause a spark ..
Is the replacement battery a good known source one, i remember hearing years ago of a guy charging up a dead dead flat battery with the polarity of the charger reversed so the Negative terminal of the battery then became the positive & visa versa once the battery was charged up, i didnt see it in person though so you never know, maybe it was just a good story ???? ..
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