If Toyota were to own 5% or more of Ford stock they would have to publically disclose the amount of their interest and also declare their intentions (investment, acquisition, etc.) per SEC regulations. I don't know if Japan would have similar regulations should Ford be purchasing Toyota stock. However, it would seem unlikely that Ford would be using needed capital to purchase Toyota stock. There is no way Ford could acquire Toyota nor is Toyota large enough to purchase Ford. A merger? It would seem unlikely that the Ford family would give up their company and although Ford is a publically traded company the Ford family retains controlling interest. Toyota is doing fine on their own and would have no reason to seek a merger. The only way Ford would seek a merger is if they were convinced that they could no longer survive on their own (the way Chrysler sought out 'Benz).
Politically it would be a bombshell to say the least.
Politically it would be a bombshell to say the least.