Location: this mission becomes available after you have completed the entire main plot and selected either Ending B or Ending C towards the end. Take control over Trevor and after you receive a message from Ron, return to the trailer that belongs to the protagonist (in Sandy Shores), thanks to which you will discover that he was visited by his mother. The elderly lady will ask her to supply her with a unique medicine.
The plan assumes that you steal the van with the Deludamol pills but, the game will not point you to the specific vehicle and require you to find one of such vehicles on your own. The Deludamol vans can be found, first of all, on parking lots at most hospitals and pharmacies in the game world. One of such places is the Dollar Pills pharmacy shown in the above screenshot, which is located to the South of Trevor's strip club, in the Strawberry district. After you have stolen the van, drive it back to the trailer, where the protagonist will not find his mother.
Rewards for the completion of the mission: none
Requirements for the completion of the mission in 100%: There are no additional requirements for this mission so, it gets completed in 100% automatically.