It doesn’t seem a good idea from the point of view of security for the bonnet to be openable by just reaching into the cabin and pulling a handle, so I fitted an electric release only operational when the ignition (or the “accessory” switch) is on.
The release uses a cheap central locking door motor. The motor is connected to pull when powered. The bonnet mechanism resets the motor back when the power is off, so a single mosfet only is needed to operate the system.
I have used the original cable and retained the original manual handle but relocated it under the car in a concealed position. This is so that the bonnet can be opened if the battery is flat or (God forbid!) the system goes wrong.
The inner cable pulls the bonnet release lever as always but the motor pulls the outer cable away from the stop (to which it is not fixed). This has the same effect as pulling the inner cable as long as the cable has some slack or curve near the end and is not fixed in position for 300 mm or so.
Door motor as advertised on Ebay for about £10
Original release cable
My favourite push button design. The microswitch gives it a very positive feel. These Chinese versions are a pound or two.
It would obviously be possible to connect a switch directly to the motor. However, the circuit shown ensures that the motor is on long enough to open the bonnet lock and also avoids damage by running the motor too long for example, if the bonnet is jammed in some way.
The circuit switches on the motor when the button is pressed. The 555 is set up as a monostable which gives an output for just over one second with the 100k and 10 micro Farad components shown. The 555 will not re-trigger until the button has been released before being pressed again.