I sold the OGTS metal fuse box when I went to a 12 circuit generic wiring system. The metal boxes work well, a bit pricey, but you will never have to buy another. The plastic fuse box would always come loose for me, so I just gave up and got the metal one. The only place I know to get the metal one is OGTS. Todd at Opels Unlimited probably has the plastic originals, but contacting him is sometimes a big deal.
Keith builds them with metal boxes at New Vintage Auto, and Splendid Parts over in Europe does the same. Either option will cost a little but you'll get a complete electrical system ready to be installed, factory correct for a GT but better than factory design. If keeping the electrical system stock is something you care about, that's the way to go. Otherwise, a universal Painless wiring kit is a good option.
I've acquired a nearly rust free and mostly whole rallye sport but i need a rear window. It had the defrost in it but i'd take one without it. I can't justify restoring it without the glass.