That relay chosen should be fine Crowbar. Also you can ignore Doug's otherwise valid comment as it's not a diode relay and doesn't need to be for your age of car. Battery polarity is irrelevant therefore.
I would disconnect the wire to fog lamps at relay pin 87. Operating your fog lamp switch then you should just hear the relay click. If this is the case you have a short to ground on the live feed to fog lamps. Carefully trace the power feed after relay to find it, maybe disconnecting at fog lamps. FWIW I would suspect issue is in one or other of fog lamps themselves - a continuity tester would reveal. Once sorted replace feed wire to relay pin 87 and change fuse to 15 amp. Do not go above 20 amp!
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