“If it wasn’t/weren’t for the coffee, I’d have no identifiable personality whatsoever” is a saying that has been printed on many images. David Letterman, host of television’s Late Show with David Letterman, said in November 1994 (as published in Esquire magazine for December 1994):
 
“Way too much coffee. But if it weren’t for coffee, I’d have no identifiable personality whatsoever.”
 
“If it weren’t for caffeine, I’d have no personality whatsoever” is a variant that has been cited in print since at least 1995. “As it turns out, 93% of my personality was coffee” is a related saying.
   
 
Wikipedia: David Letterman
David Michael Letterman (born April 12, 1947) is an American former television host, comedian, writer, and producer. He hosted a late night television talk show for 33 years, beginning with the February 1, 1982 debut of Late Night with David Letterman on NBC, and ending with the May 20, 2015 broadcast of Late Show with David Letterman on CBS. In total, Letterman hosted 6,028 episodes of Late Night and Late Show, surpassing friend and mentor Johnny Carson as the longest-serving late night talk show host in American television history. In 1996 Letterman was ranked 45th on TV Guide‘s 50 Greatest TV Stars of All Time.
     
Google Books 
Esquire: The Magazine for Men
Volume 122  
December 1994
Pg. 98:
“Way too much coffee,” Letterman says. “But if it weren’t for the coffee, I’d have no identifiable personality whatsoever.”
 
16 November 1994, Asbury Park (NJ) Press, “Names & Faces,” pg. A2, col. 6:
Other revelations: He (David Letterman—ed.) likes rock and classical but hates jazz; he hasn’t had a drink in 10 years, and before each show drinks “way too much coffee. But if it weren’t for the coffee, I’d have no identifiable personality whatsoever.”
 
25 November 1994, Calgary (Alberta) Herald, “Today’s Best,” pg. D1
As for Dave, 47, the nervous king of late night wrestles daily with his inner demons and is given to high anxiety and self-loathing, the article says. He explains his pumped-up TV persona in four words, ``Way too much coffee,’’ adding, ``But if it weren’t for the coffee, I’d have no identifiable personality whatsoever.’‘
—Cheryl Levenbrown, Knight-Ridder Newspapers
       
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“If it weren’t for coffee, I’d have no personality whatsoever.”
   
5 October 1997, Washington (DC) Post, “New in Paperback,” pg. X12:
Among other profound human mysteries, Braun considers David Letterman’s quip that if it weren’t for the coffee, he’d have no identifiable personality whatsoever and allows that, in certain ways, the talk-show host may just be right.
   
Barista Life
Barista Life’s Top 117 Coffee Quotes
Posted on Mar 16, 2017 by Barista Life
(...)
4. Way too much coffee. But if it weren’t for the coffee, I’d have no identifiable personality whatsoever.
- David Letterman
 
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“Way too much coffee. But if it weren’t for the coffee, I’d have no identifiable personality whatsoever.” - David Letterman
6:56 PM - 17 Mar 2017