I hate it when that happens. You have a short or a ground fault. Not
easy to diagnose. If you're handy you need to run over eveyrthing with
a volt meter and/or a ground wire with a bulb and find the electrical
fault. Sometimes it's the turn indicator assembly but not always.

Have you had any aftermarket electronic gear added recently?

These might help:

    http://articles.mbz.org/electric/groundtip/
    http://articles.mbz.org/electric/troubleshoot/