Expected ')' (to close '(' at line 4), got <eof> ??

This is saying that you're missing a closing parenthesis, why you may ask? You're creating a raw function to pass as the callback argument for the .KeyDown signal, which can sometimes make it harder to take in perspective in what your code is really looking like. If we collapse the line, you'll notice the code is reading:

Instance.Event:Connect(

You simply need to close the argumental parentheses. Most people just leave the closing parenthesis at function end.

local mouse=game.Players.LocalPlayer:GetMouse()
script.Parent:WaitForChild("src3")
local src = script.Parent.src3.Value
mouse.KeyDown:connect(function(key)
	if key=="h" and src.Parent.Parent.Body.headlights.on.Value == true then
		src.Parent.Parent.Body.headlights.on.Value = true
	else
		src.Parent.Parent.Body.headlights.on.Value = false
	end
end)

Your code is also pretty deprecated, meaning there are parts of your code that ROBLOX doesn't support anymore. For generic KeyBinding, we now use UserInputService. The .InputBegan signal will be what we tie a listener to:

local UserInputService = game:GetService("UserInputService")
local Player = game:GetService("Players").LocalPlayer

local Source = script.Parent:WaitForChild("src3").Value


UserInputService.InputBegan:Connect(function(Input, GameProcessed)
	if (GameProcessed) then return end
	if (Input.KeyCode == Enum.KeyCode.H) then
		if (src.Parent.Parent.Body.headlights.on.Value == true) then
			src.Parent.Parent.Body.headlights.on.Value = true
		else
			src.Parent.Parent.Body.headlights.on.Value = false
		end
	end
end)