Not sure if you have an 1973 Opel GT service manual ... this is what it says. All 1973 Opel models have fusible links located between the starting motor post and the generator regualtor. these links are the weakest point in the electrical supply system for the complete car, and, as such, will act like a fuse for every wiring harness in the car. Every accessory is still protected by a fuse or circuit breaker, of course, but fusible links protect the wiring harness before the fuses.
A fusible link consists of soldering a smaller gauge wire to a heavier gauge wire end to end. etc.
Hope this helps! I'll keep looking for the exact gauge the factory used and will let you know.
I have seen aftermarket fusible links sold at the local Canadian Tire (the Canuck version of an Automotive/Plumbing/Electrical/Sports/Gardening blah blah blah superstore), and I believe they are sold as "gauge" fusible links. That is, a 12 gauge fusible link protects a 12 gauge wire, and so on. The FSM shows links at several locations, and doesn't show a specific amp rating, but it DOES show the gauge of the wire that is being protected.
So if you look at the gauge of wire that the fusible link connects to (and protects) then ask your favourite Auto Supply store for the appropriate link, that should work.