I'm assuming that the thing makes a noise but just doesn't have enough power to open the lock. If not then it could be a duff electrical connection. Mine didn't work properly when I got the car. I took them out and opened the casings - lever them apart carefully, they are just spot glued. I cleaned out all of the little cogs and wheels that are in there and reassembled it. Now works fine.
Its just one motor - looks remarkably like the ones in Scalextric cars. The important bit seems to be the rewind when you let go of the switch. That provides some slack in the mechnanism so that when you pull the lever next time, the motor can get up to speed before engaging with the door catch. I think that when they get clogged up with crap over time, the wheels do not spin freely enough on the way back and therefore the wind-up time, and resultant power, for the next opening is reduced.
You do have to take out the back seat pad and interior trim but its not too bad a job - 15 mins max, mainly torx screws. Inside, the servo is held onto the door catch by a couple of bolts - not hard to get out.
Alternatively, if you don't fancy the DIY challenge of refurbishing the servos, I don't they are that expensive to buy new.
Mine failed too. Easy job to change it as Mitchella says.
When mine failed, the motor could be heard whirring, but was not making the "complete sound". If I remember correctly the motor was not that expensive.
Are they both (driver & passenger) not working BTW? If one is working, why not swap it over just to be sure.
I found that by taking the central locking mech. out and applying some WD40 in the shaft it mede it work lovely. mine were sticking when i bought the car.