Latest update for Garmin GPS 100?

I have an old Garmin GPS 100 that I'd like to put the newest possible Jeppesen update on. I believe the last supported update was in 2000. It's kind of a long story as to why I want to use this antique, but it's in an airplane and it would be about $15K to upgrade it after all is said and done.

I can make a data cable easily and I could even design a microcontroller interface if need be (I'd prefer not to). I could build my own update from publicly available information if I knew the format.

Can anybody point me in the direction of documentation / software / database archives / etc. that I can use to update this unit?

Thanks.