There are 2 "illumination" bulbs. If 1/2 your cluster is dim, it would suggest that one of them is inoperative. They are located approximately in the area of the turn signal indicators and feed light into clear plastic light diffusers, which spread the light out on that half of the cluster.
If neither of the bulbs seem to be working, i would agree that the dimmer is often the problem.
If the cluster lighting seems to be too dim (but operating) on BOTH sides of the cluster, are you sure the dimmer switch is turned up to full brightness? I've had this problem presented to me a number of times where the dimmer was simply turned down too far (accidentally I suppose) and the owner didn't know that the dimmer switch was even a control that they could use!