I was pulled over by an undercover police car last night.
I was stopped by a unmarked police cruiser just outside my house.
この場合、次のように言えますよ。
ーI was pulled over by an undercover police car last night.
「昨夜、覆面パトカーに車を止められた」=「つかまった」
an undercover police car で「覆面パトカー」
ーI was stopped by an unmarked police cruiser just outside my house.
「家の外で、覆面パトカーに止められた」
an unmarked police cruiser で「覆面パトカー」とも言えます。
ちなみに、私服警官のことは a plain clothes police officer と言います。
ご参考まで!