“Honk if you love Jesus” was a popular bumper sticker saying from 1971. The bumper sticker spawned lots of other “Honk if you…” imitations, but one notable “Honk if I’m…”
 
The “Honk if I’m an Aggie” joke bumper sticker dates from at least 1990. Even some Aggies display them!
   
       
19 March 1971, Pasadena (CA) Star-News, pg. 7, col. 1:
Bumper stickers call for a response: “Honk if you love Jesus.” 
 
Google Groups: rec.humor.funny
Newsgroups: rec.humor.funny
From: .(JavaScript must be enabled to view this email address) (Brad Templeton)
Date: 18 Mar 90 08:25:08 GMT
Local: Sun, Mar 18 1990 4:25 am
Subject: More of the Oneliner file
 
I saw this one on a bumper sticker a couple days ago:
Honk if I’m an Aggie
     
Texas Wit & Wisdom
by Wallace O. Chariton
Plano, TX: Wordware Publishing, Inc.
1990, 1992
Pg. 146:
HONK IF I’M AN AGGIE
The All-Time Best Aggie Bumper Sticker. 
 
Google Books
God in the Stadium:
Sports and Religion in America
by Robert J. Higgs
Lexington, KY: University Press of Kentucky
1995
Pg. 214:
“Cow colleges” and “football factories” became interchangeable. Clemson and Texas A&M, it seems, have had to bear most of the burden of much of this humor—for example, the bumper sticker “Honk if I’m an Aggie”—but it abounds everywhere rivalries arise between the land grant school and either an older military order—VPI versus VMI—or a private school—the University of Tennessee versus Vanderbilt.
 
Kinky Friedman’s Guide to Texas Etiquette:
Or, How to Get to Heaven or Hell Without Going Through Dallas-Fort Worth
by Kinky Friedman
New York: HarperCollins
2001, 2003
Pg. 39 (Seen on Texas Bumper Stickers):
Honk If I’m an Aggie