Hi, coolfly!
Below, is a draft of such a letter that you might use as an example. You could tailor it to suit your needs.
Dear [the person or people to whom it will be addressed],
Please accept this correspondence as an indication of my interest in obtaining a promotion to the position of [ ? ].
In support of my application for the position, I submit the following to show how my experience qualifies me to be considered for the position.
(You then would go on to list your qualifications and experience.)
I look forward to an opportunity to discuss how I might best serve the needs of our company in the position for which I have applied.
Sincerely,
[Your Name]