How to script Position.Magnitude in an if statement?

What I'm trying to do is that if the car goes forward, it goes back to it's normal speed and when it goes in reverse, it changes the value.

local seat = script.Parent
local car = seat.Parent
local M = seat.Position.Magnitude

local LD = car.RL.WheelMount.Drive
local RD = car.RR.WheelMount.Drive
local LS = car.FL.SpringMount.Steer
local RS = car.FR.SpringMount.Steer

local MS = seat.Speed.Value
local SA = seat.Angle.Value

local function onChanged(property)
	if property == "Throttle" then
		LD.AngularVelocity = MS * seat.Throttle
		RD.AngularVelocity = -MS * seat.Throttle
	end
	if property == "Steer" then
		LS.TargetAngle = SA * seat.Steer
		RS.TargetAngle = SA * seat.Steer
	end
	if M > 1 then
		MS = 77
	end
	if M < 1 then
		MS = 30
	end
end
seat.Changed:Connect(onChanged)