Well, I did things a little out of sequence. I wrote a complete script (179 pages) for a prequel/sequel to the movie Christine, then contacted the producer of the original movie. After a couple of e-mails, he replied and politely but firmly told me it is currently way too expensive and complicated (several entities own rights to the franchise) to produce a sequel. He would not even read my script because he has been bombarded with them and he thought most of them were not good, so he has stopped accepting them.
This was another time where it would help to have an agent, but I have been unable to find one, yet, as people here are probably really tired of hearing me constantly whine about.
I thought this roadblock would happen, but I felt so strongly about this project that I went ahead and wrote it anyway, if only just to clear my tiny little brain of the story. Being a huge gear head and classic car and music fan was a big factor, too.
I have no regrets about writing it, and I strongly encourage you to go forward with your plans. Life is way too short to not follow your dreams.