"I used it with a "mallet" style steel head hammer. The word "tapping" does not really apply. Give it good, solid hits and keep at it."
Heavy hammering +aluminum =not good.
Try saturating the offending part with Kroil or second choice PB Blaster,multiple times.WD40 is proven ineffective in penetrating oil comparisons on Bitog.
While you are in there ,remember to check the suspect "Y connector" Pelican sells it for about $60.
here is a dwg:
BATTERY CABLE for 2002 Porsche Boxster|986-607-019-02