dear robo,
there is no front fog light *idiot* light on any model car. If you buy a front
fog light switch from the dealer it may have a small green light built into it
that will turn on when the lights are on. It would be hard to install a factory
switch and make this light work. It will be hard for your stereo guy to make connections to this switch becase it would normally have a specific volvo wire harness plugged into the back of it.
As far as buying "volvo" relays, forget it. Wiring up fog lights is a piece of
cake, just have the installer wire them so they only go on with the head lights
and high beams. meaning the switch that will turn them on will only work if the
head lights are on. this way you can't forget to turn them off, they will automatically shut off with the key off.