If it's like my 1991 Pulsar, do this:
1. Get the code from your dealer. Mine was "2420" (might be the same for all 1991 pulsars?)
2. Turn key to first ACC (don't start the car, cos that's a waste of petrol)
3. have the radio on, wait till it stops and asks to the code (3min)
4. Hold down "4". A digit should come up.
5. Use tuning up and down to enter first digit.
6. When entered, use "5" button to enter next one...
- Continue until all four digits are in.
7. It should work...