“Honk if (you see) parts fall off” is a jocular saying that has been printed on many bumper stickers. “I saw a funny bumpersticker on an old pickup truck yesterday:  Honk If You See Parts Fall Off” was posted on the newsgroup bit.listserv.dorothyl on September 24, 1996. “I’m adding this bumper sticker to one. It reads, ‘Honk if parts fall off’” was printed in the Seattle (WA) Times on January 22, 1998.
   
   
Google Groups: bit.listserv.dorothyl
Abbott/D’Amato/Off-topic cat story
baty
9/24/96
(...)
PS—I saw a funny bumpersticker on an old pickup truck yesterday:  Honk If You See Parts Fall Off.
     
16 November 1997, Wausau (WI) Daily Herald, “Bumper-sticker Wisdom” by Pat Peckham, pg. 6D, col. 1:
“Honk if you see parts fall off.”
   
22 January 1998, Seattle (WA) Times, “Eastside’s Manly Mates Ready to Lug Their Share” by Shery Grindeland, pg. B3:
I’m adding this bumper sticker to one. It reads, “Honk if parts fall off.”
 
Google Groups: alt.motorcycles
Tire Made In Argentina
Ed Mills
8/7/98
Howdy:
 
I have a medium-ugly GL500 for which I bought a cheap new front tire, the reasoning being that I didn’t want to spend more on it than the funky old thing is worth.  Also, I needed a little money left over from the tire purchase so I could get a “Honk If You See Parts Fall Off” sticker for the rear fender.
   
1 December 1998, Oshkosh (WI)

, “Bumper stickers are on the decline” (AP), pg. C3, col. 5:
Among those he (Phred Bishop of Phresh Stickers in Portland, Oregon—ed.) considers his greatest hits: “Nixon in ‘98: He’s rested, he’s ready, he’s dead,” and “Honk if you see parts fall off.”
   
Google Groups: rec.autos.makers.vw.aircooled
Bumper Stickers Seen on a tricked out Honda
RC5CAR
3/1/00
One of the best…
HONK IF PARTS FALL OFF
       
17 July 2000, St.Petersburg (FL) Times, “Citrus Slices,” Citrus Times sec., pg. 6, col. 1:
BUMPER STICKERS WE LIKE: Honk If Parts Fall Off.
 
Google Books
Life’s A Gas:
Foibles, Faux Pas and Social Blunders
Compiled by Ken Alley  
Lincoln, NE: Writers Club Press
2001
Pg. ?:
HONK IF YOU SEE PARTS FALL OFF
 
Google Groups: novell.community.chat
Seen on a Bumper Sticker…
G
12/4/01
Honk if parts fall off car.
   
Google Books
Risking It:
A Romantic Comedy
By Angela Quarles
Unsealed Room Press
2006
Pg. ?:
He’s wearing a trucker cap with the words “Honk if you see parts fall off.”
 
Twitter
shelley
@shelleygwen
Behind an old beat to hell vw with a sticker that read “honk if parts fall off”
9:13 PM - 3 Jun 2008
   
Twitter
Ren
@the_ren
Bumper sticker on car infront of me says “Honk if parts fall off.” Time to switch lanes.
11:26 AM - 8 Dec 2008
     
Super Street
1990 Nissan 240SX S13 - Licensed To Ill
Tommy Cho’s S13.5 Is Dope And We’re Puttin’ It On Wax Paper!
Sean Klingelhoefer –  Nov 30, 2009
(...) 
To make sure he didn’t have to place a “honk if parts fall off” bumper sticker on the car, Tommy got with ARC and GReddy to combat any oiling issues with the use of their oil pan and filter relocation kit respectively.
 
Twitter
Resident Nomad
@resident_nomad
The bumper sticker: ‘Honk If You See Parts Fall Off’
10:49 AM - 12 Mar 2011
   
Twitter
Jared Davis
@texasfireman06
“Honk if parts fall off” 😂😂 @ Austin, Texas https://www.instagram.com/p/BseL9ykgogLHwA4OKPyU3rpO-QDxwi_OoZFltM0/?utm_source=ig_twitter_share&igshid=6t1a6yjtstxg …
5:59 PM - 10 Jan 2019 from Austin, TX
 
Twitter
Karmin
@beingkarmin
Honk if you see parts fall off! #MyBumperSticker
8:21 PM - 5 Feb 2019