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)

It's because you didn't close the bracket for the function like this

local mouse=game.Players.LocalPlayer:GetMouse()

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

mouse.KeyDown:connect(function(key)
    if key == Enum.KeyCode.H and src.Parent.Parent.Body.headlights.on.Value then
          src.Parent.Parent.Body.headlights.on.Value = true
    else
        src.Parent.Parent.Body.headlights.on.Value = false
    end
end)
--Done