Coastal Dravidian route:(Tulu->Malayalam->north-western Tamil Nadu)
alveolar approximant r -> alveolar trill/tap r (Tulu)
alveolar approximant r/alveolar trill/tap r -> retroflex approximant r/retroflex palatal approximant r -> palatal approximant y (Malayalam,Tamil, few other SD-I tribal languages)
Correction to the following:
During SD-I and SD-II split, alveolar approximant r, might have given rise to an allophone alveolar plosive d. Which became retroflex plosive in SD-II languages. But I think this should have happened with a big chunk of SD-I speakers too.
alveolar approximant r -> alveolar plosive d -> retroflex plosive D
I think it should be;
alveolar approximant r->retroflex flap r->retroflex plosive (another group of allophones)
On a side note, this is what I found about Burrow, Emeneau and Krishnamurti's Dravidian etymology Dictionary.
Our understanding of Dravidian is strongly related to the Dravidian etymological dictionary of Burrow and Emeneau (1984 and online). However, this is very Tamil-centric and the literature constantly confuses its head entries with proto-Dravidian (e.g. Krishnamurti 2003).
My understanding is that Tamil literature might have preserved the words that would have lost in other languages. Also, it might have preserved the cases and other grammatical structures that are no longer used. However, when it comes to consonant sound changes(vowel sound changes aren't all that important), the old Tamil literature cannot be referred as the sounds are native to a geographical region.
Commenter Varttik (who doesn't agree with this anyway).