I think you're right. The bolt heads may be covered with grease. Should take a look from the inside of the wheel-well <img src="/forums/images/graemlins/cyclops.gif" alt="" />, clean-off the grease and expose the bolt heads. Also, take a look at the replacement caliper to see where the mounting bolts s/b, <img src="/forums/images/graemlins/kewl.gif" alt="" />