As
maviczap says.If you use a high powered one watch the current draw.
I have an older single Stebel on a CX500 which didn't need a relay as it came as a pair which would have needed a relay.
I'd take a punt on one of these:Advertised @115 Db.
£5.95
http://www.ebay.co.uk/itm/12v-Replac...cAAOSwmLlX83AI
HTH