this certainly sounds like a low battery voltage so if you're sure that the battery is in fact fine ( have you tried a jump start from another car?) then I'd remove the engine bay fusebox lid and ensure that its all clean and dry in there
then double check the main battery connections for tightness and cleanliness
if these are OK then trace the battery earth lead back to the engine / chassis and confirm that this end is also tight and clean and if this isn't the engine earth connection then locate the heavy wire that earths the engine too and check this too
the starter should be located on the back of the engine block just under the exhaust manifold
this image shows where it bolts up
that method of checking current drain is exactly what I'd recommend to narrow down what circuit is giving the issue
A typical current drain with everything off and un-alarmed should be around 0.05Amp and if its higher than this remove a fuse and re-measure, if measurement doesn't change then replace the fuse and remove the next.
do you still need help?
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