Opel GT Ignition Switch Diagnosis:

The Opel GT Ignition Switch is prone to internal resistance buildup & premature failure, due to a primitive circuit design which routes up to 30 amps from the battery through it, when the starter is cranking. (In the mid-1960's, Opel rummaged through the parts bin, using the switch from the Opel Admiral, and some parts from French sources. In quality terms, think: Renault). Typically, there's a puff of smoke, and you're stranded. To help prevent this, the Opel GT Ignition Switch can be tested, either in the car (using key to turn) or off the car (using a screwdriver to turn), with a simple ohm meter.

Testing the Opel GT Ignition Switch (On the Car):

(1) Disconnect the battery ground cable
(2) Follow the wire loom from the steering column to the white plastic plug on the side of the fusebox, and carefully remove plug from the (brittle) fusebox.
(3) Use a common volt-ohm meter, to test the switch in the "start" position. Connect one test lead to the end of the solid red wire and another to the end of the striped red/black wire. (See: "Fusebox View")
(4) Rotate the ignition key, to the start position and hold. The ohm-meter reading should drop from "infinity" to a steady 4 ohms or less on a good switch (less than 1 ohm indicates an excellent switch). Spray-cleaning the switch also lowers resistance, (requires partial disassembly of Steering Column).
(5) Release the switch for a moment. The ohms should revert back to infinity. If it doesn't (if measurement goes back and forth), the switch may have poor internal circuit isolation, which indicates carbon buildup or overheated parts. (If so, spray-cleaning may provide a quick-fix).
(6) You can also test the non-starting power circuit, by connecting the test leads to the solid red wire and to the solid black wire. They should read less than 1 ohm in the "on" position and infinite ohms resistance when "off."

Testing the GT Ignition Switch (Off the Car):

Use a phillips-head screwdriver to carefully rotate the "cross-shaped" center of the switch, between the  "off" "on" & "start" positions, to perform the (#4-#6) above tests of resistance and isolation. Spray-clean with WD40 around "cross-shaped" area, rotate switch a few times, and let dry. Retest switch,  to verify measurement of starting circuit of 4 ohms or less. You can also inspect the switch, to see if the striped red/black wire appears melted, or if the metal housing has a "bulge" around the "cross" area (indicating heat). If needed, an Opel retailer now provides rebuilding services for about $50.00. But when the white plastic backing has cracked, the switch has broken, and is non-repairable.

Opel GT Protective Ignition Relay installation:

If the ignition switch is verified to be in working condition, the first protective action recommended is to install a protective 4-prong 30 amp relay in the starter circuit. An inexpensive relay will take up the brunt of the current when the starter is powered, keeping it out of your valuable original GT ignition switch.

The most readily available is the Radio Shack #275-226 (costs about $6).

The circuit schematic for installing this relay is as shown below.

To install, you will need to assemble a  harness, with 3 short wires, 4 clip ends and 2 eyelet connectors, to connect to 3 relay terminals, 1 solenoid terminal, 1 round solenoid battery post, and a (suggested) round ground on the chassis. The existing ignition connection will be transferred from the starter solenoid to the new relay (per schematic).

A good place to mount the relay is above the starter, on the under-hood side engine area.