They are probably more likely to respond than to write a letter back to you. Members on this forum have had responses using this method – some have even had responses whilst the presenter is on air.
The address would be:
I wrote a letter to a presenter a few weeks ago and am still awaiting a reply. Do I have much chance of getting one?
It is highly unlikely they will respond to a letter by post unless it is something important–what sort of stuff did you mention in the letter, was there any specific questions as such?