The output of the Prop-1, Prop-2, Prop-SX, and EZ-8 is the ULN2803 which, for all intents and purposes, behaves like a normally-open switch that connects to ground. So, you can use the terminal block output of any of these controllers to start the CAR/P-300.
1) Use one output and V+ to connect to the V-Trig input on the CAR/P-300
2) Set the CAR/P-300 input voltage for 12v
3) A short pulse on the output will activate the CAR/P-300
Easy-peazy.