Rhotacism is a defective utterance of the "r" sound. Ironic that the word itself begins with that exact sound, eh?
Anyway, I've been aware for a long time that my "r" can sound a little like a "w". I've noticed it most on my song recordings - it's not major but I can hear it...
So, a couple of days ago I thought I'd have a Google and see what I could find...and...I found a cure!!
I'm 38 and I've only just learned how to pronounce the letter "r"!!
You don't really use the lips, this is what leads to a "w"ish sound. I found this: you make an "l" sound with your tongue touching the top of your mouth and then you just slide the tongue back wards while sounding and it become an "r" sound! I was amazed! A revelation!
I also found a drill to train your tongue to perform this new trick. You find a word with "tr" in it and replace the "r" with a "d". Stay with me here. So let's use "strumming" - you say "stdumming" over and over and the transition becomes natural after a time and it becomes the correct "r" sound.
For me, singing is all about confidence, and I have reached a level of confidence with which I'm happy - but - the "r" thing was in the back of my mind nonetheless. So, now I have a solution to the pronunciation problem I am OVER THE MOON :-)