swithes and relays and wires
Any switch on the dash of the Opel needs a relay to handle the power of the headlights. Period.
There is already a capable relay in place, it uses the signal from the rollover microswitches to activate it, and it then supplies power to the headlights. The easy thing to do would be to change the input signal to this already existing relay to the foglight switch instead of the microswitches.
The reason the GT uses the miroswitches is so the headlights can't come on in "down" and overheat themselves. I've personally never seen a GT have problems with failed microswitches causing the lights to go out. Then again, I don't usually work on GT's and I've probably only worked on 10-15 of them.
The reason GT's usually have problems with the headlights is due to the wires themselves. There are 3 wires going to the headlight of an Opel, white and yellow for high and low beam, and a color I can't remember for the ground (probably brown.) Because of the flexibility requirements necessary for the flip over headlights, the factory equipped the car with wires that were insulated with ruber instead of PVC like most wires on the car.
Have you ever seen a 30 year old tire that you would want to use?
EVERY GT needs to have these rubber wires replaced. Yes, all of them. It can be a real pain in the arse, but you have to do it. Once the wires are replaced, there is a couple of quick adjustments to make sure linkages all line-up, and making sure to lube the cable thoroughly, and the headlight system is relativly trouble-free. One of the downloads available here, in fact, is a pretty darn good "article" on maintaining the headlights that every GT owner should read.
About the wires. There are 2 types of wiring commonly available to the consumer these days. One is insulated with PVC, and the other with teflon. The PVC wires run on the order of 4-15 cents a foot, and the teflon for around $2 a foot. For your GT's headlights you really want the Teflon, if at all possible. Luckaly, almost every automaker switched in the early 90's to teflon insulated wires (easily identifiable by the insulation being almost 1/4 as thick) so you can save some $ wth a good pair of snips at the local junk yard. Get the headlight wiring from a Honda and you can leave the ends on the wires where they connect to the bulbs and use them too. Once the wiring is replaced, the headlight system on the GT is not any more "dangerous" than any other auto.