I have 2 (one gang) on/off light switches in my lounge (one at each end of the lounge). I'd like to change one of them over to a (one gang, two way) dimmer switch. One back box has lots of wires - I think the wires are going in and out of there. The other back box has fewer - I think this is perhaps where the circuit ends. Which switch should I replace with the dimmer switch please or does it not matter? Thank you.