Some of the most vocal objectors to the $1.6 billion settlement with Toyota Motor Corp. have agreed to drop their challenge in exchange for $1.5 million to research electronics systems used in cars.

Two objectors to the settlement—backed by the Center for Auto Safety, a Washington nonprofit founded by consumer advocate Ralph Nader, and the Consumers Union—had argued that a $30 million auto safety research fund established by the settlement essentially endorsed Toyota’s position that drivers, and not defects in the electronic throttle control system of their vehicles, were to blame for accidents. That dispute lay at the heart of the litigation.