86 Honda Prelude

I bought my son this car a few months ago. The previous owner put in a new alternator days before we bought it. Within a week or so the car wouldn't start, we put in a new battery. Shortly after it died while he was driving, we had the alternator tested, came back bad we replaced it. The problem has continue, last week we replaced the alternator (third time), external regulator and the battery (second time). Now the lights and radio are beginning to dim again. Any ideas would be most grateful. It is an 86 Honda Prelude with 92000 miles, 1.8 liter.