My bonnet switch is unplugged as I've removed the 2nd skin on my bonnet. All I did was hold the pin down, set the alarm, then removed the wiring plug. I'm pretty sure it makes the circuit complete with the switch on the bonnet in the open position. Try disconnecting the plug and setting your alarm mate.
Other than that, how many terminals are in the plug? 2 or 3? If there's 2, bridge across them with a piece of wire (assuming the alarm switch needs to see a complete circuit to allow it to set of course). If there's 3 I'll have to look on my own car and work out which one does what. Try the top option first though.