You can buy "behind the mirror" led arrows for this.
http://www.aliexpress.com/item/2pcs-...887380825.html
Will look like this:
I am planning to do this, but not as a turn signal, but a blind spot warning with sensors, because the Alero has huge blind spots to be honest.
(I don't work for Alibaba/Aliexpress and I am not affiliated with them in any way)