Simply take one of the following cars from the Power and Glory pack, as an example we'll use the BMW M3 Rally, which is an 80's Rally car.
Choose the Kenya Rally from within the DiRT Tour year 1 - Rockstar Transworld Clash event (it's the second one across) and choose the BMW M3 Rally. Before starting the race you can set up the car (bottom option), simply slide one block from out of the middle and press 'B', complete the race. This will pop after completing the first race, you do not need to complete the whole event.
This will also class as one of the uses of a Power and Glory car for the "New Wheels" achievement.