Taxiway diagram -
http://155.178.201.160/d-tpp/1513/00282AD.PDF
Rumor is they landed 20R and went B to T3 and wound up in the ditch at T4 & Juliet.
At the link below scroll down to "RAW: Surveillance video of plane" to see the accident (I wish they had not edited it) -
Southwest plane goes off taxiway after landing in Nashville | WKRN News 2