My Back wheels are squeaking on my 53 model mondeo 20. TDCI?

My back wheel sare sqeaking on my ford mondeo, i dont now why this is. It usually starts when the vehcile has been driven for several miles. i dont know wether its caused through the brakes semi locking on or tight bearings, i know how to adjust the brake cable, but i havent got a clue how to adjust the bearing, i have looked at the end cap withe the wheel off but it looks closed to me, is there anyone who can help