I had the same gauge problem with my 300D. The fuse was visually intact and undamaged but I replaced it anyways. It fixed my problem and cost $0.50. I would say it is worth a try.
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K