I wondering about an add-on proximity sensor for the alarm...anybody know of one?

 

You can add one from any one of the aftermarket alarm companies. All you need is 12v power, ground, and then there is a trigger wire off the sensor. Based on the sensor it will either be a positive 12v trigger or ground. If you tie it into the door switch to ground the switch it will replicate the door being opened and trigger the factory alarm. You will need to find a switched 12v lead that is active once you lock the car but not when driving (this might be available at the tow alarm switch but you will need to check) or place a switch in the car so you can power it up when you need it.