i have a 90 sportster and i noticed the other night when i put my blinker on the headlight dims as the blinker lights up.the bike runs good and no starting problems.

 

DO you have a voltmeter on the bike? run the bike up to around 2000rpm and put a meter on the battery. You should be getting around 13.5 - 14V DC.

Alternator/voltage regulator going bad and outputting just enough to keep the battery from going dead maybe.

 

Check the ground for the headlight. Sounds like you have a loose or dirty ground connection.

 

Check all the connections in the bike. I agree that it sounds like a corroded connection, but if the light system is showing signs of age, other systems are as well.
...gene