The Declasse Asea is arguably the rarest car in GTA V. Although it has only marginal performance, it is able to be uniquely modified using the "Sticker Bomb" graphics. (The "I'm Not A Hipster" DLC adds the Benefactor Panto, which also features the "Sticker Bomb" mod.) As of now, it is only found in one instance in the Story Mode and the begining race of GTA Online. (EDIT: The Asea also appears as Michael's rental car when he returns to North Yankton and pursues Trevor to the graveyard during the mission "Bury the Hatchet", but in this case the car obviously can't be saved during game play.)

For Story Mode you obtain it by reaching the end of the Strangers and Freaks mission "What Lies Beneath". In order to access the car you must kill Abigail before she gets in the car, after that car will be unlocked and you will be able to take it. In order to get it in GTA Online follow the tutorial in the video below.The default version of the Asea from the first race is obtained by selecting it and getting to the part where you have to apply an insurance and tracker to it as described in the video. Don't bother trying to get the "Sticker Bomb" version of it because it has been patched. People who had it their sticker bomb car replaced with a another one and put into the cheater lobby for a limited time.

How to get the StickerBomb car (no glitch/locations) from Business DLC