ground could have been bad from the start , i assume there was contact at the bolt on the seat where you connected your ground . but the resistance was high enough to cause stress issues in your wire heat ,burnt ofc or cca whichever you use. bad ground would work but would not last long. due to high resistance . let me give a example. i bolted to a seat once and when car was started the trunk battery only read 13.1-12.8 . front battery was 14.4+ . my ground was nice and shiny 2 . but the resistance was so high it was struggling to get a good connection. i relocated my ground to a strut bolt which is connected directly to frame . back battery now reads 14.8+ . hope i could help . if i was you i would relocate ground.
p.s. people recommend ground to be no longer then 12inches or whatever . the reason for this is to reduce the chance of engine wine or rca interference . .ground can be rather long with no issues