Just looking for a little advice. I am currently refurbishing a catering trailer and am at the stage of rewiring. I am going to install this alarm system - https://www.ebay.co.uk/itm/Wireless-GSM-SMS-WiFi-Smart-Home-House-Office-Security-Burglar-Alarm-Systems-Kit/123502533704?ssPageName=STRK%3AMEBIDX%3AIT&var=424051663112&_trksid=p2057872.m2749.l2649
I was just wondering if it would be possible to "splice" a nano, say into the alarm siren cable to flash the sidelights in case of activation. Sidelights will be powered by a 12v battery and alarm and nano from a 5v supply from the battery too.
Thanks 
You can not drive a 12V signal direct with a nano. You will have to use a transistor between the output and the 12V load.
Or, you could use a tap off of the siren drive to trigger a NE555 timer to flash the lights and save a few bucks... (and a poor defenseless Nano)
Diving the 12V side lights is the easy part But I guess you also want to light them when the trailer is behind a car? So try to sketch (pen and paper will do fine, please no Fritzing mess) how you are planning to wire the trailer lights and the alarm + battery without trying to link the alarm to the sidelights.
And, for the linking to an Arduino:
- Any power (battery life) requirements?
- what output levels does the alarm return?
- and what does the Arduino need to do? Only flash?