Volvo 960 drivers door wont unlock

[jmarvinr]

I recently purchased a 1992 Volvo 960. It has a problem in that the drivers door will not lock or unlock with the key. From inside the car you can push down the drivers door lock button & get normal locking / unlocking. Is the problem likely the key or something with the lock, or the power locks ? Seller said he thought it needed a "new switch".
Thanks.