Rockenbach was sold in 2013 and changed the name to Rock Auto. There are still employees there that worked for Rockenbach, ask for John Shaw, hes' been there a while: (847) 223-8651
Next contact the Grayslake Historical Society and ask for Charlotte Renehan:
http://www.grayslakehistory.com
Click on the contact tab for her numbers, says they have an extensice archive there. I'm sure they have plenty of info on Rockenbach Chevrolet, looks like they opened their first garage in 1926.
Paul