Go west, and into the southern doorway. Head east, and then south. Run west, and go north until you find a switch. Flip it, and enter the western room. Grab the red key, and return to the start of the level. Go east, and then north. Enter the passageway in the northeast, and open the red door. Go around the bend and grab the yellow key. Open the yellow door once the walls have lowered, and activate the lift. Take it up, open the next door, and flip the switch.
Other points of interest
Directly in front of the red key is a switch. Flip it, and a wall will lower in the southeast to reveal a teleporter. Enter it to be warped to a small compartment with a chainsaw. (sector 97)