508D Sensor in Astro Van

Ok, this place has been a great help with my directed alarm and remote start install, thank you! I have two questions I have the 5806 clifford model alarm and I want to install the 508d proximity sensor in my van. My biggest concern is the roof rack near the rear of the van easy access able via the bumper. Can anyone suggest the best location to install the sensor? I was thinking of near the middle of the van in the headliner but what about this ground plane business? Or middle of the van on the floor below the sleeping area? The manual says I need to provide a separate constant 12 volt and ground since the unit cannot support it on the 508d, will splicing in longer trigger wires affect it at all? Thank you

Sensor needs to be as low as possible to work correctly. You might want to consider 2 508d units to get even coverage, As adjusting to cover in larger vehicles can cause false triggers as the 508d senses in a circular area. Adjusting to cover the whole van front to back will make the sides over sensitive. Ideal placement for 2 of them would be center of each half of the front and back with a over lap in the middle. Length of wires is not a concern as the trigger is a pulse not a constant feed on the trigger wires.