The only thing I knew was wrong with my 2001 vert when I bought it 2 months ago was the passenger door lock would every now and then automatically lock when it was supossed to but it would never unlock (manually it was fine). I assumed it was the lock actuator, found a used one on Ebay for $30 shipped, installed that (not too hard) but that didn't fix it. I then figured it had to be the door module. After looking at tons of used ones on Ebay I decided to go with this guy who rebuilds them:

http://www.ebay.com/itm/1997-to-2004...#ht_2027wt_689

I sent it off (USPS flat rate for about $5) and got it back in a week. It works perfectly! It is a very easy part to get to if you can get the door panel off which is also pretty easy. If you have funky door locks I recommend this guy!

 

Glad you got it fixed! Its a relay inside that fails over time. They actually aren't difficult to fix yourself if you can use a soldering iron.

 

Well Damn! I should have done more research. I am familiar with a soldering iron, I just assumed it was a more complex problem. Oh well, it's fixed and an whole lot cheaper than the dealer would have charged......

 

Its still nice to have it fixed! If it fails again pull the module out and open the case. You'll see 4 relays on the circuit board. They are: window up, window down, door lock, door unlock. I don't know which is which but there is a diagram out there somewhere, or you can carefully plug it in and listen to which relay clicks in response to which button to figure it out.

There are 2 things that can happen, if you are lucky the relay might have just worked its way loose due to years of summer heat and winter cold, it may just need the solder joints reinforced on the bottom fo the board. Or quite likely one of the relays has gone to St. Peter, use the part number on top and go to mouser or digi-key to find a new one or cross referenced equivalent. If you are handy with a soldering iron remove the bad relay and put a new one on (this is almsot certainly what the repair service you used did anyway).
You'll be back in business for only a few bucks and an hour of time spent, and have a great feeling of satisfaction that you fixed it yourself!

There are quite a few circuit boards in the C5 that can be fixed by either reinforcing solder bonds that have come apart or replacing a simple component like this on the board,
Digital HVAC dim display - resolder some resisters
Door lock intermittent - replace relays
Power seat SCM problem - replace relays
Key fob keeps loosing sync with car but the battery is ok - reinforce solder bonds on the pads of the surface mount IC inside the fob.

There are of course plenty of things that can cause parts to go bad, but lots of times the above is all that's required.