Below is a complete walkthrough to help you beat doors & rooms 3-1.

1. First pick up the cleaner inside the box.
2. Use the cleaner to clean up the license plate of the ambulance.

3. Use the code 5728 to open the lock in the back of the ambulance. Obtain a crow bar.

4. Use the crowbar to open the front door of the ambulance. Start the car engine. Note the color in the back of the ambulance: Yellow, Green, Red, Blue.

5. Unlock the door with the code Yellow, Green, Red, Blue

Speed Note
You can pass this stage quickly by using the code Yellow, Green, Red, Blue on the lock.