Complete ALL of the following:
Stop and get a good view of the fireworks in Brighton.
(I got this in Career, in the Fan Favorite section, Race #3. Use the H2 (Off-Road) and right after you pass the 'start' line stop on the incline of the hill facing the fireworks (make sure to NOT go on one of the 2 paths around it). Also, right after you get to the Ferris Wheel bear LEFT and sit on top of the hill for a few seconds, and this should pop up. Alternatively, you could just stop somewhere on the track and use the camera to get a clear view of the fireworks and that MAY work if the other method isn't working for you.)
Drive a smoking car through the oilfield in Amboy.
(@ Badlands Traverse track, car needs to be in Critical Health (one Red Bar, Black Smoke) you can use forward mines to damage yourself if you're not in critical yet. You may not even have to drive through the oilfield itself, but just drive near it on the race course for it to unlock.)
Nitro through the Amboy gas station.
(@ either Route 66 or Badlands Traverse tracks. Depending on which track you play on there is room to slow down before you get to the gast station so that you can line up the Nitro acceleration.)
Clean Nitro through the Hollywood sign in online Multiplayer.
(This is done on the map Hollywood Hills (Downtown Vista) - Go on the leftmost path between the letters - you will most likely have to fly off the edge, be sure to not hit anything while going through. - thanks to Qwerty P D4NG3R for confirming!)
*The road on the left side of this screenshot is where you need to go.*
*HERE is the loading screen for the track.*
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