black97max

Has anyone else experianced this problem? My back seat door lock seems to be broken. Sometimes it unlocks sometimes and sometimes it wont (Majority of the time it wont. When it doesn't open sometimes the motor makes a grinding noise for a couple of seconds. I had an aftermarket alarm/keyless entry installed a year ago (not sure if this is connected to the problem. Does anybody know what this problem could be? And how much it will cost me to fix? All advice is appreciated.
Thanks