Is the buyer of a car which is subsequently stolen is relieved from his obligation?

No. Private respondents are not relieved of their obligation to pay the former the installments due on the promissory note on account of the loss of the automobile. The chattel mortgage constituted over the automobile is merely an accessory contract to the promissory note. Being the principal contract, the promissory note is unaffected by whatever befalls the subject matter of the accessory contract. (Perla Compania de Seguros vs. CA G.R. No. 96452 May 7, 1992)