negative ang una kasi po pag accidentally nasagi mo yung wrench sa body ng car OK lang, hindi mag-short kasi negative din yung car body (- to - so no electrical flow). pag positive inuna mo at nasagi mo yung wrench sa car body e di short circuit ang labas nun (+ to -), sira paint ng car mo saka bungi na yung iyong made in germany na 12mm wrench sayang naman. ALSO, since sabi mo nga pwedeng ikaw yung "ground", hindi ka makoryente kasi negative to negative yung hawak mo (car body and negative battery terminal). imagine mo kung reverse ito, positive terminal ang hawak mo tapos grounded pa din (meaning connected pa sa negative terminal ng battery) ang car body e di electrocuted ka. (OK OK! 12 volts don't kill. but some people do get a shock from a 12 volts DC battery)

pag install naman ay reverse. positive muna. since di pa connected yung negative terminal kahit maisagi mo yung wrench sa car body wala pang electrical circuit so walang short circuit. di ka din makoryente kasi di pa connected yung negative terminal, hindika pa "grounded" kahit nakahawak ka sa car body.

"ground" is a misnomer for negative terminal so don't be confused by terminologies. there is also such a thing as "positive ground", like some old old cars.