With an open top sports car should the inidicators have and audible tick when on. Should this be heard over the engine road noise etc?

IF SO ..... Does anyone know where I can get a 12v FOG horn and how to wire it up to ?

Its not an SVA requirement

the requirement is for audible or visible so the warning light is sufficient

We got an audible bleeper from Maplins and wired it into the indicator circuit. It can be heard over the engine and road noise (at reasonable speed) The visible warning light aint so visible in direct sunlight and the ticking can't be heard at all.

Straight forward job to wire in a bleeper - they only cost pennies and work well