How do I make it so I can detect a second key press of the same key?

Well first let's add a variable that either says the lights are on or not

local lights=false

Next let's add it to the start of your script

local car = script.Parent.Car.Value
local mouse=game.Players.LocalPlayer:GetMouse()
local lights = false
mouse.KeyDown:connect(function(key)

end)

Now let's check if the lights are on or not if on do this if not then do that

local car = script.Parent.Car.Value
local mouse=game.Players.LocalPlayer:GetMouse()
local lights = false
mouse.KeyDown:connect(function(key)
	if lights==false then
		
	elseif lights==true then
		
	end
end)

Now let's tell it what to do if the lights are off, also in the loop we will check that lights is still == to true

local car = script.Parent.Car.Value
local mouse=game.Players.LocalPlayer:GetMouse()
local lights = false
mouse.KeyDown:connect(function(key)
	if key=="e" then
		if lights==false then lights=true
			while lights == true do
			        wait(1/27)
			        car.Body.RIGHT.Material = "Neon"
			        car.Body.RIGHT.Transparency = 0
			        wait(1/27)
			        car.Body.RIGHT.Material = "SmoothPlastic"
			        car.Body.RIGHT.Transparency = 1
		        end
		elseif lights==true then lights=false
			
		end
	end
end)

Now what happens when its false

local car = script.Parent.Car.Value
local mouse=game.Players.LocalPlayer:GetMouse()
local lights = false
mouse.KeyDown:connect(function(key)
	if key=="e" then
		if lights==false then lights=true
			while lights == true do
			        car.Body.RIGHT.Material = "Neon"
			        car.Body.RIGHT.Transparency = 0
			        wait(1/27)
			        car.Body.RIGHT.Material = "SmoothPlastic"
			        car.Body.RIGHT.Transparency = 1
				wait(1/27)
		        end
		elseif lights==true then lights=false
			car.Body.RIGHT.Material = "SmoothPlastic"
			car.Body.RIGHT.Transparency = 1
		end
	end
end)