How do I weld bricks together?

How do I weld bricks together? I need to weld Police lights to my police car, but everytime I put them on, they fall off. How do I weld them?

local weld = Instance.new("Weld", game.Workspace.Part);
weld.Part0 = weld.Parent;
weld.Part1 = game.Workspace.Part2;

I might add this for easy use:

weld.C1 = part1.CFrame:inverse() * part0.CFrame

"part0" being the weld's Part0, "part1" being the weld's Part1. Adding that will position the parts in the correct fashion.