It seems like the number of campaign sound trucks asking for people's votes has gone down.
There seem to be less campaign sound trucks going around asking for people's votes now.
解答1より解答2の方がカジュアルな言い方です。「__は減少しているようだ。」は “There seem to be less ___.” と言います。例えば “There seem to be less bugs out today because it’s windy.” (風が吹いていますから、いつもより虫が少ないようです。)選挙カーは “campaign sound truck” と言います。