One of the Nests in the Berley Lake Infestation Zone is found here so you’ll come across this Checkpoint while doing that activity. After clearing the freakers and the Nest nearby, recover the NERO Fuse from the container inside the tunnel’s maintenance shed.
Replace the fuse of the Breaker beside the MMU’s door then activate it. Disable the speakers on the roof of the MMU. There are only two speakers so cut them off.
Find the gas can near the guard booth in front of the tunnel. The generator is located behind the fenced area. Load it with gas then power it up to access the MMU.