Hi ! Here's all the step you need to know.
1. Go to Sunrays Apartment 2nd floor and go in the room 201
2. Then go talk to Regina until she talk about the "Thunder Bitch"
3. Go back in your truck and end the investigation (no need to banish the ghost there)
4. Next you need to go in Raven Household (By putting Cummed Water on the power Breaker of Sunflower city streets and end the investigation)
5. Go to the Raven HouseHold map, and go to Regina room. look into the desk and you'll fing clues about the thunder girl, then end the investigation again
6. Go back to the room 201 on Sunrays apartment again, and talk to Regina so she ask you to help her catch the thunder girl.
7. Go to room 202 and interact with the Special Ghost Lamp (the flower on the table), a little thunder girl will appear.
8. End the investigation and go back once again in the Sunrays Apartment. It is 100% the thunder girl.
9. Hunt the thunder girl then once you fucked her, you'll find the elder thundergirl roaming around the map, follow her until she go into room 202.
10. Then use the special rope to catch her in the room 202, leave the room and enter again to have the animation and get the "Special Ghost Lamp"
The special ghost lamp is a very powerfull item that let you choose the ghost you want to hunt before choosing your map. It will also help you with the "doctor" quest ^^