Nissan Repair: alternator/electrical surges, nissan sentra, electrical surges

Question
I have a 93 nissan sentra
I had to replace the alternator a year ago and bought a new battery
I have had wires that connect to the headlight wire harness corrode and break and the headlight fuses melt

I recently had the new alternator power cord melt and short out on the exaust manifold and the alternator itself, i repared the wire and now the seems to have electrical surges.  The dome lights, headlights, and electrical stuff is getting brighter and then dim bright dim bright dim....
But, it doesn't do it if I put enough things on the rear defrost the radio and headlights and it doesn't get brighter.  

Answer
John,

It sounds like the rectifier on the alternator shorted when the wire melted to the manifold.  You still get a charge but the oscillations are caused by one of the diodes being blown.  You could buy a rectifier and change it out or change the alternator.