While traveling to the border in Road 96, you will wind up at different spots, including Biggy's Diner. Here, you can break into a couple of rooms to get some good items, but you will need to complete some objectives first. For instance, you will need to acquire the Hacker ability.
How to get the Hacker ability in Road 96
To get the Hacker ability, you need to encounter Alex during the Beep Boop Boop mission. Once you get to the mission, Alex will talk to you and play a game of pong with you in the diner. After the game, Alex will say to steal back some money from the diner. You will get the Hacker ability and break into the back room. After helping Alex, you can go to the employee room to open the safe.
This safe does not seem to have a combination that can be found by looking around the area. Only after getting the Hacker ability will you be able to get into the safe. It is possible that there is a code for the safe, but using the Hacker ability is much easier.
Depending on when you get to the Beep Boop Boop mission, there will be other safes and doors that will be blocked off. If you can make your way back to these locations, you can open previously locked locations. One spot this is possible is at the Big Bear Diner. The back room can be pick locked or opened with the key near the shop owner. The safe inside can also be hacked and does not seem to have a code that opens it.
Road 96 is an interesting narrative title that will place you in some unique situations with procedurally generated interactions. Each playthrough is a little different, so you may have a different route you take to reach the end.
For more Road 96 guides, check out What is the safe code at the Happy Taxi station in Road 96? on Pro Game Guides.