Is the only way to do so is to make an onTouch to the seat and check if the player's Humanoid Sit == true?
Assuming the script is in the seat,
script.Parent.ChildAdded:connect(function(weld) chr = weld.Part1.Parent -- That is the character that is sitting in the seat. plr = game.Players:GetPlayerFromCharacter(chr) -- This is the player that is sitting in the seat. -- put code in here end)