lights failure

since i had this warning come on and replaced the high level brake light unit, which was apparently defunct because it now works, i have had this warning come on repeatedly yet there are no lights out, and i have fiddled with the rear ones several times because i know that these can be a constant source of problems, but there is only one bulb that appears to be slightly loose in the holder, and after manouvering it sometimes works until the car hits a pothole then the warning comes on again, any solutions anyone?

any idea how much a pair of replacement rear bulb holders will be?