I just wrote some instructions in another thread as to how you can access the back if yours has the fake foam bricks around it. It is not hard, but it is not really a trivial job. If it is not the heater fan then my guess is that it is the rod with the foil that turns to simulate the flames and you can access this from the back. Here's a link to the other thread and it is the second post.
//heartlandowners.org/showthread.ph...replacement?highlight=replace+fireplace+bulbs