adding the back door to the alarm

Hey guys I've spent the day repairing the locks on my new (to me) defender and sorting out the alarm so it works again on the front doors and bonnet.
When I came to do the rear door the switch was there but their is no wiring. Am I correct in thinking if I take a feed from one of the front doors to this switch it will simply just add it to the alarm?? Or will this cause any problems.

Yes it will, it will also put the interior light on when you open the back door

My lights don't come on with the doors. It only works if I manually turn it on. Will check it out tomorrow. Thanks