PIR, Ultrasonic, Microwave, RF Field Motion Sensors for car

Hi,

i want to change the original car alarm in my 91 Calibra cause it can’t do much.
The original alarm has 4 Ultrasonic-Sensors under the roof in the middle oft the car
on the left an right side (see attachement). On each side one points to the back and one to the front.
So i guess one car side is sender and one receiver.

I can’t find the specs of the sensors with the number on them: C40-16K PM 108
But they have the same size like: http://www.voelkner.de/products/385540/Ultraschall-Sender-KPUS-40T-16T-K768-40-kHz-x-H-16-mm-x-12-mm.html

Then i like this Dual Zone Motion Sensor: http://www.amazon.com/Directed-Install-essentials-Motion-Sensor/dp/B0009VFOME/

And i found this Microwave Sensor: http://www.amazon.com/SMAKN-Microwave-10-525GHz-Doppler-Detector/dp/B00FFW4AZ4/

Is the Microwave Sensor the same thing like a RF Field Disturbance Sensor?

The Problems i see is rain and flies.
People wrote that the Dual Zone Motion Sensor is reacting to rain.
What about ultrasonic and flies in the car?

I was reading about something called “Cross-Zoning” - two sensors/zones need to be activated at nearly the same time in order to trigger the alarm.

Some ideas about preventing false rain and flies alarms?