Everything electric goes on and off
I drive a 1983 300SD with 300k miles. Driving down the highway yesterday, my radio quit playing. I then noticed that the speedometer and all gauges did not work, the blinkers, windshield wipers, headlights, taillights, etc did not work---nothing electrical worked. Several miles later, everything began to work again. When I got home, I pulled the battery out, tested it, found it to have no charge---completely dead, so I replaced it with a new battery. I had installed a rebuilt alternator two months ago. After installing the new battery, the same thing occured with the electrical system going off within one mile of driving after I had installed the new battery. Where should I start to troubleshoot the problem? What are the most likely things that this could be?