New alternator but amp meter showing negative

Hi all,

I'm looking to purchase a V8s this weekend which seems to have an issue with it's amp meter. Whilst the new alternator is charging (i know this by testing the battery with the engine running with a multimeter) but the amp meter is showing a negative reading.

Can anyone suggest what might be happening? I read somewhere that sometimes the polarity of the amp meter needs to be changed? Whilst i'm not overly bothered, I like things to work how they are intended...