A complete search of the internet has found these results:
Thanks for the heads up is the most popular phrase on the web.
Both of these terms have the same meaning and are used to thank someone for letting them know about a fact or situation in the present or future. "Thanks for the heads up" is the more common colloquial expression." "Thanks for a heads up" is not used nearly as often, but could be said or written in specific cases depending on the context.
3,044,979 results on the web
Thanks for the heads-up, Earl.
Okay, thanks for the heads-up about the meeting.
Thanks for the heads-up last night, Janet. It gave me some time to plan my night.
Thanks for the heads up, Artie. See you at the softball game on Friday!
Sorry I missed your call, Mr. Coleman, but thanks for the heads-up.
Although it could be too late to change plans, thanks for the heads up.
Thanks for the heads up. I haven't had time to fix myself up.
Thanks for the heads up on the credit card usage--I didn't know I had spent so much!
Thanks for the heads-up, by the way.
Thanks for the heads-up on the eyewitness who saw the redhead.
"Thanks for the heads up," said the man, after his friend told him that he had jury duty the following week.
320,883 results on the web
Thanks for giving me a heads up on this project.
Okay,** thanks for a heads** up regarding the house inspection.
Thanks for giving me a heads up last night, Janet.
Can I give you a heads up on what she is going to ask you?
Sorry, Steve, you didn't give me a heads up on this matter earlier.
Thank you for giving my mom a heads up about the family gathering in advance. I appreciate it.
I gave you a heads up but you didn't listen and jumped in the frozen lake anyway.
Thanks for a heads-up on any future plans we might make.