light on dash for belt tensioners- vauxhall astra- 2004 1.6?

my belt tensioner light on dash stays on,also my heated rear windscreen button is permanantly on and my rear fog lights do not work, when I press either the fog light switch or heated rear windscreen screen switch it cancels out the belt tensioner on dash, i have bought a new relay switch and also a full unti that controls heated rear window and air con etc but did not correct the fault- do you have any suggestions?