2001 Vauxhall Zafira heater and doors

If thermostat stuck open , engine runs cold and never reaches normal temp .Could check for broken wiring where it passes from door pillar into doors , quite common .

How would I check the wiring in the door for the heaters? The heater does not warm up.

your door central locking for both rear doors is a wiring fault not the locking device in order to locate the problem correctly go to the car put your head to the door handle at the rear door and click lock you will hear the locks kick in however the rear doors will not of locked so open the door and look down to the door mounts you will see a rubber going from body into door its secured by a blue clip squeeze together it will pop open peel back rubber and check all wiring garuntee its snapped close to plug the door to frame are male to female which just pull out get an auto electrician to use heat seal wire reconnectors cheap but very effective to recouple wire or goto vauxhall for modification kit 100.00 when you see what it comprises of your option would of been my way new wire half foot long with reconnector for inside car rim male and female and a new plug for door to body then you have to rewire it or pay vauxhall the rip offs to do it trust me if you know an auto electrician its easier effective and doesnt break again had mine done chuffed to hell saved me an absolute packet