adding trunl pin and sensor

Yes, just tap into the same wire for the hood pin switch. If you want to add another sensor make sure you diode isolaet the trigger wire so there is no feedback from the other sensors' trigger wire.

Usually it's the blue ( but make sure you check it ). It will be the wire that is ground until you trigger the device and it will pulse 12 volts. Use a 1 amp diode inline of this wire and have the band facing the device.

Your sensor would actually rest at an open circuit and would pulse ground when triggered. The diodes band facing the device is correct that would allow ground to travel from the sensor to the alarms negative input wire. It is best to also diode isolate the hood and trunk triggers as well, especially if the trigger also turns on a hood or trunk light . This would prevent the trunk light from coming on when you simply just open your hoiod or vice versa. also prevents damage to the sensor. Most sensors have a 300mA or less negative ground output and trying to light a trunk bulb that may draw 1 or more Amps would eventually damage your sensor