re wiring fog lights?

Ok so all i want to do is make my fog lights come on when i use my high beam. I also want my fogs to continue working as they do at the moment. Ie i have low beam on and i can switch fogs on via the switch (after modifying them i don't want the main beams to come on every time i use the fog switch). I've asked this question on a few forums but no one has come up with an answer although i think it should be quite simple.

Could i split the positive feed to one of the main beams to go to a relay and then take the power out from the relay to the two fog light positive wires. Would i need to fit a diode to each of these positive wires, got a feeling i should but not sure why.

Any help on this would be appreciated and please make answers as simple as posible so i can understand it rather than just going out and wiring something up following instructions. I like to know how and why something will work.

Oh and i did do a search but couldnt find anything. 

What kind of car are you working with?

Do you want the fogs on EVERY time you have the high beams on, or only if the high beams are on and the fog light switch is on?

Make sure you check your local laws regarding making modifications like that - some places have really weird laws regarding when fog lights can and can't be on.

Sorry i should have said i'm working on a 1992 toyota mk2 mr2 but assume the wiring for this would be the same whatever vehicle it was being done on.

I want the fog lights to come on everytime i use the full beam/flash headlights whether the fog switch is on or off but i still want to be able to use the fog lights via the switch the in the same way they can be used now.

Don't worry about any of the legal side. If i was making this so they came on with a dipped beam then i might have problems but with main beam the only problem might be service time but i could just disconnect this wiring for that time.

I, personally, would just add a relay. 

Connect the + side of the coil of the relay (pin 86) to the high beam trigger wire.  Connect the negative side of the relay (Pin 85) to ground.

Connect pin 30 (common) to the battery through an appropriately sized fuse and then connect Pin 87 (normally open) to the fog light wire(s). 

The relay will provide isolation so the switch will work as normal as long as the high beams aren't on.