I've assembled the last bits and it's working! :)
First I've cut the power and data lines on the RF module of the alarm so it won't draw unnecessary current and to allow the new module to drive the microcontroller input pin.
Then soldered the Groung, +5V and data wires to the corresponding places. The power pins on the capacitor at the output of the regulator and the data on a via near the PIC.
Lastly I've glued the wires to the board with hot glue so they wont get easily disconnected.
After this I assembled it on the car and voila! It works.
But then I found that only one of the signalling light output relay is working! Forgot that this gone bad just before the RF module gave away. And worst, it's the driver side that gone bad so it's harder to see when it received the command signal. I'll address this later.
The main issue that I found (and I was sort of expecting this but hoping it won't be an issue) is that the range s#cks! It's 1, 2 or at best 3 meters, depending on the angle horizontally and vertically. This is because the receiver sits inside the dashboard and the doors shield the signal. Because the cable is a bit short I can't rise it more (my fault!) but I'll get an extension or solder a new one. I'll try to put it on top of the cluster, this way I'll get a clear view of it from almost all angles and maybe this will solve the problem. Must make some tests.
I'm evaluating the possibility of discarding the microcontroller on the alarm board and all the remaining circuitry (ultrasound drivers, etc) and connect the Nrf pins to the relay driver and implement the alarm functions on it. This because the standby current is about 20mA! This is way too much for me. I suspect that replacing the 7805 for a more efficient DC/DC converter will make this better but if I get the PIC out it would be even better :)
P.S. 1 Lacks a video of it working. I'll address that soon...
P.S. 2 The key fob has arrived! Must have a look at it.