Not sure if all this applies to 1999 models, since 1999-2002 saw many electrical changes. I'm basing this on my experience with the 1990-1998 models.

If you have had any moisture or water issues, definitely check on as many grounds as you can. Very prone to corrosion. Also, the seat control units are in a recess on the floor under the seats, so can have water pool there (not sure who thought having an electronic module be on the floor, in a swimming pool-like recess, in a convertible that might get rained in...). Make sure to check the fusebox in the trunk, too (right side, towards the top, when looking in from the open trunk lid). Have seen lots of corrosion issues with the fuses in the trunk from moisture vapor, also affecting the unit under the trunk panel, next to the spare tire.

I wish you good luck. Electrical gremlins can be a nightmare to track down in an otherwise perfectly good car. Given that it is a 99 with the newer modules, maybe a good scan tool will be able to help trace back to a problem source.

-Bruce

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1976 240D [W115.117/616.916] (acq 11Jun76)
1990 MB 300D 2.5 Turbo [W124.128/602.962] (acq 4Sep15)
1991 SL250D (ex-300SL-24) [R129.061/OM602.962/722.618, was M104.981] (acq 25May12, converted to diesel)
1993 Jeep Cherokee [XJ] (acq 12Apr14)
2013 GLK250 BlueTec 4MATIC [X204.904/651.912] (acq 24Jan26)