can i splice in wires from the bootlight and from the main interior light together or will they mess up each other?

my alarm came with a piece of wire in a 'Y' shape that has thick black connections where it joins together. i think it has diodes or some sort of resistors in it. what is this for, its not shown in the alarms wiring diagram?

my alarm came with a dual stage shock sensor, i also bought an upgrade of a dual microwave sensor. i'm going to use the microwave sensor but would like to know if i can maybe use both. the alarm only has one socket - they could share the same earth and +12v wires but what about the input and output wires?