Jim,
Take the seat lock assembly apart as much as possible removing all the obvious screws. If I recall correctly, two philips head screws hold on a plate which when removed allows access the latch and spring that engage the seat plunger. After removing the spring and latch mechanism there is an arm/cam which pulls the latch back when the key is turned. Behind this cam is a little cir-clip that is tucked in and a little hard to see; this is what holds the lock mechanism in place. It is a good idea to clean everything up first and use good lighting. A small jewler's type screwdriver or similar tool can be used to remove this cir-clip. Once this cir-clip is removed the lock assembly can be removed from its housing.
Once the lock is out it will look something like the picture of a disassembled seat lock on the following website.
http://home.earthlink.net/~cmiddleton01/removingabmwairheadforklockThere is a thin metal (brass or copper colored I think) that is pressed into place. Under this plate are the pins and springs which are the heart of the lock. There are two pins and one spring in each of the three holes. Removing and discarding these pins and springs will allow the lock to operate with any key or a screwdriver.
If you want to re-key the lock to fit you existing ignition key you can experiment with replacing a different combination of pins in different holes until you find a combination that allows the lock to turn only when the key is inserted. Sometimes you can take out one or two of the three sets of pins and create a lock that will work with your key. A lock with only one pin will still work like a lock, it is just likely to be opened with a variety of different keys and is very easy to pick.
Some of the above description is done from memory and might be in error. Questions or problems send me a message. If you really want help, sent me the seat lock with a key you would like to work in it and I'll fix you up. (You get to pay the postage.)
