Before you dismiss the cosmic ray theory, consider that the issue has been known since the 1950s, and airplane and spacecraft manufacturers design in safeguards that triple-check all data as a defense from such interference from space. Later, in the 1970s, researchers found that small amounts of this radiation does indeed make it down to the surface and can cause problems with small electronics like cell phones and computers.
For its part, Toyota responded to requests for comment from The Detroit Free Press that its electronics "are not the same as typical consumer electronics. The durability, size, susceptibility and specifications of the automotive electronics make them robust against this type of interference." Sounds like Toyota isn't buying the cosmic ray explanation.
Apparently, there's just one sure way to know if there's any legs to the issue of cosmic rays causing unintended acceleration, and it involves placing errant vehicles in front of particle accelerators and showering them with radiation. It seems that there are a number of firms that can perform such tests, but it remains to be seen if Toyota will make its vehicles available – and, we assume, write out a rather large check – for such testing.
[Source: Detroit Free Press]