That is the correct way to wire them and you can have more than one because my highway tractor has about 20 circuits wired like that. The relays run on switched ground. The only difference I see tho is the primary side of the relays on mine get their power from a separate circuit on its own 5a breaker. Don't know if that matters or not. I think whatever it is it will be ground related. When you step on the brakes and the dash lights come on it usually means the brake lights lost their ground and draw back thru the tail lights which will light up the dash.

Kevin