Typically, you would wire the trunk light positive to the alarm. Sometimes, that wire is available in the front. Vipers(and all DEI alarms) usually have a "hood/pin" input for this. Should be labeled on the brain of the alarm. You should identify the correct wire on the trunk lid using a multimeter before connecting it to the alarm. If you dont have a trunk light, you need either a pin switch or a mercury switch, as posted above. Pin switches usually rust over the years or get beat up. Mercury switches can be unreliable, like when you park on a hill, but are easy to install. You would wire either of these into the same spot on the alarm brain - "hood/pin".

Good luck