You could have gotten this information in any number of places like the Faq Thread or FAQS..but..
To get the Maintenance Tunnel Key:
When your game starts, go into Leisure Park. Walk to the Northwest corner of the park where the white pavillion and flowerbed area is at. You will see a walkway path leading into that wall in the northwest corner. Follow the path to a parking lot where you will see a red convertible and a motorcycle.
Get into the red convertible and drive into the Maintenance Tunnels (entrance is there at that parking lot).
When the Maintenance Tunnels load....take a right...then a left...then a right.
If you look at your map, it's basically a big T underneath the center of Leisure park. That dead end is the Maintenance Tunnel Warehouse.
Go into the door and you will see the Maintenance Tunnel Key and one of the respawnable SMG (Sub Machine Guns) in both far corners of the Tunnel Warehouse. There are seven SMG locations around the mall.
As far as other Maintenance Tunnel Questions goes:
There are 2 driveable vehicles inside the Maintenance Tunnels itself. One driveable van is underneath Seon's Grocery Store and one white sedan underneath Paradise Plaza. This means that a player can go back and forth from Seon's to Paradise Plaza underneath the mall.
The rest of the Maintenance Exits do NOT have driveable vehicles underneath them.
Anytime you exit the Maintenance Tunnels and come back in, a new car will have respawned in those 2 locations.