Carolyn Monaco. Only she had both the key and the opportunity to gain access to the victim’s car trunk between weekend jumps and tamper with the cord.
The murderer must have used a key to unlock the car trunk and tamper with the cord Saturday night. Rita deMont probably had a key to her husband’s car, but she had a substantiated alibi of being out of town that night. Frank Marron might have been able to visit the car after the party, but there is no reason to believe he had a key. Carolyn Monaco, however, returned to the area that night and could have copied the key while she had borrowed deMont’s car.
Carolyn, who had had an affair with her boss and then been dumped by him, had in fact committed the crime. She thought the mostly internal incision in the cord would escape deMont’s notice (which it did) and later be mistaken for a sign of a faulty cord (disproved by the cleanness of the incision and the metal traces).