Before proceeding,
I will list the four possible scenarios:

1. New game
   
2. Existing game WITHOUT the map mod
   
3. Existing game WITH the map mod
   
4. Existing game to which I want to add the map mod

In this case, no additional actions are required. Simply add the mod along with any vehicle mods you wish to use.
It is always recommended to use the auto-arrange feature for the mod list to prevent issues.

Requirement 1 (MANDATORY): Delete the map_basements.bin file from the folder where the saved game is stored.

Requirement 2: If you have NOT visited the area where the interiors spawn, no further action is needed. If you have visited that area, you must delete the generated chunks for that zone
Image from mod zone: https://imgur.com/MPrxJuJ.png
To delete the map chunks in the specified area, you can use my tool[buchojefe.github.io]
(I recommend setting the zoom to 50% when using it).

Step 1: Follow the instructions provided in this link.
Step 2: Delete the map_basements.bin file from the folder where saved game is stored.