I had the same scenerio happen to me. I replaced the voltage regulator and it seemed to be okay for a couple of days. When all the lights lit up again I pulled into an auto parts shop to have the battery and alternator checked. It turned out my alternator was toast.

From what I have read, it is usually the regulator and I was not so lucky.

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1993 190E 2.6 135k
1989 Ford Thunderbird SC 5 Spd 79K