Grand Marshal

Ok. I'm not sure if it's supposed to not have a horn but it does not. Even when I try to change to a different horn it is completely silent. If someone knows, let me know please. Thanks.

 

Mos likely hydraulic suspension controls overwrite the horn key ("H"), try to add another binding for horn
Same happens with bluebuck

 

there actually is a horn for the gm, but as diamond coal said, the lowrider hydraulics overwrite horn. this happens to the bluebuck too.
best thing is to rebind horn or lowrider (horn is easier) or just live with it