1990 Opel Corsa Opel Corsa/Vauxhall Nova gsi rev counter pr

I fitted a gsi clocks to my corsa last year connected up rev counter and all was working good, but last week the rsv counter half works when the lights are turned on only, the needle revs up and down between 2000rpm and 4000rpm freely wen it wants, when the lights are turned off it works ok