I realise this isn't a Lumina forum, but I figure a '99 Lumina is not so different from a '98 Malibu and maybe this will help someone here.
My elderly M-I-L has a '99 Lumina that had problems with the power door locks. The switches on the doors wouldn't open them, neither would the keyfob. I think the driver's door would lock, but none of them would unlock...something like that.
Anyway, I traced the problem to a relay buried behind the glove box. When a door switch was actuated, the relay had voltage going in, but little or no voltage coming out. I replaced that relay and for $21.79+tax I now have a very happy mother-in-law.
I disassembled the old relay and found it was full of carbon dust and the contacts were all burnt and pitted, so even though it still "clicked" when actuated, it was bad.
I don't know if the Malibu uses the same relay, but I bet it does, so before you take your car to the dealer and empty your bank account, check that relay.
Very useful and timely information. I'm sure we're going to see a lot of door lock problems as the weather gets cold. This does apply to the gen. 5 Malibu except that the door lock/unlock relays are in the driver side instrument panel fuse block.