I just joined this forum to share my experience with you. I had a very similar problem on my 1976 280Z (all original) two weeks ago. In addition to the symtoms that you listed the turn signals didn't work either. I consulted the Datsun workshop manual and it pointed towards the ignition relay, which is common to all five circuits. There is also one additonal common component, and that is the 10 Amp fuse for the fuel pump, which as I discovered upon troubleshooting yesterday was open. Not sure if this will help, but if you haven't checked the fuse yet, then I would start there. The fuse on my car is located in the fuse box located on the passenger side under the glove box.