Hwy 101 north of Florence or south of Yachats to the parking lot for Devils Elbow State Park and Heceta Lighthouse. Follow the trail up from the beach past the lightkeepers house to the lighthouse.
The trail behind the lighthouse is not recommended due to a sharp dropoff on the back side to the crashing waves below! If you go, make sure you don't go alone -- but the payoff is usually worth it.
Hiking up the "trail" behind the lighthouse in the gathering darkness is not recommended! But it got me this photo which is one of my all-time favorites. The Heceta Lighthouse is the most photographed lighthouse in North America, so you have to find unique visions of it to stand out from the crowd, and I hope this photo meets that need.