Green’s Dictionary of Slang

ding-ding wagon n.

[echoic of its bell + prob. ding-ding n.(though records fail to back this up)]

(US) an ambulance used to convey psychiatric patients.

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[US]Fort Wayne Jrnl Gaz. (IN) 19 Nov. n.p.: Look out lady or the ding-ding wagon will back up for you, and they’ll put you in a cozy corner.