Unknown Electrical System Issue

93 Lexus GS300 Two months ago while driving the red battery light on my dashboard lit up, and my car stereo and powersteering went out, but my car continued running and I was able to drive the car a mile home. The next day the car ran fine. The day after the issue happened again. This time my car died while parked running, and I couldn't get the car to jump off. I took the battery and had it tested, the said it was bad and sold me a new battery. Well, it worked for 3 days then the same thing went down. Couldn't get a jump. They replaced the battery for me, and tested out my alternate, batter, and starter. All tested out fine, they concluded I had a bad ground somewhere. I replaced my negative battery cable because it was corroded pretty bad, and this seemed to remedy my problem as the car ran fine for a month. Well, the car has failed again. I don't have the money to take it to a mechanic right away, and I feel like this could be something simple. Any ideas or suggestions? Thanks in advance for your help.