What is this thing???

When I reupholstered the seat in my 1974, I reconnected the wires coming from the seat switches. Not long after that, I got into the car after work and it wouldn't start. No click, no nothing. Eventually I guessed what was going on, removed my bag from the passenger seat, started the engine. After that I disconnected the wires again, just like the original owner probably did on day one.