300 SDL Guages suddenly stopped working

1986 300 SDL 110K miles. Today I noticed a slight hum coming from the area above the steering wheel. The oil pressure, temp, and speedometer guages all are suddenly not working.
Also, the glow light indicator and sound don't come on when starting the car. Otherwise, car drives fine. I had a new radio put in a couple of days ago but this is probably not related, although I asuume it is an electrical problem. Can anyone tell me what is wrong? Thanks,
-- Jeff
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Check your fuses....one of them is cracked. It may be #9 but I am not positive. It is the one for the instruments.

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Fuses are all ok. However, the buzzing noise is coming from a black box 1" x 3" in the fuse box which says 12V/2(4)x21W. I looked it up on mbz.org and it says multi-function relay for blinger, hazard, wiper and rear window heater. Doesn't say anything about gauges.
Here again are the symptoms: all the gauges (oil, speed, temp) not working. Relay in fuse box is buzzing (blinker relay). Blinkers work and buzzing stops when signaling left or right turn. New radio installed last week (@ Circuit City), but symptoms appeared only several days after.
My guess is that the multi function relay is ok, but there is a short circuit somewhere causing it to buzz as well as causing the gauges not to work. I'll see my mechanic later this week but has anyone experienced similar problems? Thanks,
-- Jeff
snipped-for-privacy@mit.edu (jeff) wrote in message

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Get a test light, connect the clip end to a good ground, turn on the ignition, check each fuse for 12v at each end at the brass terminal. Both ends will light the test light if it is a hot circuit and the fuse is good. You will find a cracked fuse.

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