Simple Anti-Theft Switch, Wiring suggestions Options

[FlatSix]

So I want to install a switch hidden under the dash to stop the engine starting.
Having done a search, the black/purple wire going to the tachometer seemed a likely candidate to break the circuit. However, when I disconnected it from the tach, the car would still start!
Which wire should I use?
The engine is a 1973 2.0 four cylinder with fuel injection.

[FlatSix]

Some really interesting suggestions here, thank you all.

Being in the UK, the manual gearbox is not a problem and even the selector pattern is easily overcome. The car will pull away in 2nd gear and reverse can only be in one of four places! LHD is problematic when overtaking but not an anti-theft deterrent.

I'm liking the ideas of either cutting power to the fuel pump or using the interlock device. I also like the idea of easily unplugging any changes I make and returning it back to stock should I need to for troubleshooting etc.

I apologise for being somewhat economical with the truth earlier. Whilst the engine is from '73 the car is actually a Californian spec '76. Therefore there is a kinda interlock system under the passenger seat. This is much simplified for '76 as there are no pressure plates in the seats. The car runs fine without the relay in place. All the relay seems to do is illuminate the fasten seat belt sign for a few seconds. I believe the yellow wires are bypassed at the factory. I don't want to put a switch in line with the yellow wires as they look to be taking a fair bit of current. I'm not sure how I can take advantage of this with a switch under the dashboard though.

[ClayPerrine]

Some really interesting suggestions here, thank you all.

Being in the UK, the manual gearbox is not a problem and even the selector pattern is easily overcome. The car will pull away in 2nd gear and reverse can only be in one of four places! LHD is problematic when overtaking but not an anti-theft deterrent.

I'm liking the ideas of either cutting power to the fuel pump or using the interlock device. I also like the idea of easily unplugging any changes I make and returning it back to stock should I need to for troubleshooting etc.

I apologise for being somewhat economical with the truth earlier. Whilst the engine is from '73 the car is actually a Californian spec '76. Therefore there is a kinda interlock system under the passenger seat. This is much simplified for '76 as there are no pressure plates in the seats. The car runs fine without the relay in place. All the relay seems to do is illuminate the fasten seat belt sign for a few seconds. I believe the yellow wires are bypassed at the factory. I don't want to put a switch in line with the yellow wires as they look to be taking a fair bit of current. I'm not sure how I can take advantage of this with a switch under the dashboard though.

The yellow wires are not for the fuel pump. They connect the ignition switch to the starter to make the engine crank. If you interrupt that, the engine won't crank over, but the car could still be push started.

I once put an anti-theft switch in a 280Z for a customer. I put a button under the carpet between the clutch and brake pedals. You had to put it in neutral, and the step on the right spot on the carpet to get the engine to crank. The customer quickly got used to it.

For a 914-4, you can ground the tach wire on the back of the tachometer. Or kill the fuel pump, or something similar.

There are also GPS Trackers. https://www.amazon.com/GPS-Trackers/b?node=617650011 With one of these, just let them steal the car. then go get it back when it stops moving. Or notify the cops and let them get it back for you.