Yes, really. Rodents.
Back in 2016, a class-action lawsuit was filed against Toyota claiming rodents, such as rats and mice, were causing damage to vehicles by eating the engine wiring insulation. The suit claimed the engine wiring insulation is soy-based, a change from the petroleum-based material (specifically vinyl chloride) the Japanese automaker previously used, and something rodents evidently like to chew on. This results in, quite clearly, damaged engines that make the vehicles, such as the Toyota Tundra, incapable of being driven.
Toyota isn't the only automaker who switched wiring materials for environmental reasons, but it continues to be a target of angry owners. Despite a court dismissing the case in 2018, according to Car Complaints, a new lawsuit is fighting the court dismissal.
At that time, the judge ruled the soy-based wiring did not constitute a "latent defect." For its part, Toyota argued that rodents have always been pests known to chew on things and, simply put, it's just a reality that's always been there. The plaintiffs recently and successfully appealed to the Court of Appeals for the Ninth District in California which has reversed in part the US District Court for the Central District of California's earlier ruling.
The appeals court determined the class action "directly support Appellants' theory, which necessarily excludes Toyota's explanation." However, the appeals court did not overturn the previous court's ruling that Toyota fraudulently failed to disclose the alleged defect.
The plaintiffs have so far failed to convince any court Toyota was aware of this issue beforehand. The appeals court found that the district court incorrectly identified rodents as the alleged source of the defect. The real source is the soy-based wiring harnesses that caught the rodents' attention (and appetite) in the first place. Therefore, this alleged defect existed at the time of purchase even if the damage occurred at a later date.
Where will this go next? It's now up to Toyota to respond and we have no doubt it'll once again argue for a dismissal.