advertising truck; billboard truck
A truck or van equipped with loudspeakers or large signs, used for commercial advertising or political campaigning.
During the election period, advertising trucks drive around the city.
That advertising truck is playing music for a new product at high volume.
街宣車 specifically refers to a vehicle used for street propaganda, often by political groups, while 広告宣伝車 is broader and includes commercial advertising.
Compound of 広告 (advertisement), 宣伝 (propaganda/publicity), and 車 (vehicle).