Well see, the thing is, that sometimes when the battery terminals or cable connections are corroded, they will cause ghost electrical problems. Often intermittently.
On another hand, have you been paying attention to your electric gauge while your turn signals are causing the dimming?
Do you have a multimeter/voltmeter? If so, try reading and following this simple diagnostic procedure. It doesn't cover everything, but it'* a good start and will cover 90%+ of most issues:
TheCarConnection Tip: Solving Electrical Problems