You need to buy a digital voltmeter. As others stated, very cheap at Harbor Freight etc.


***First, to test your glow plug relay. Turn car on to glow as normal. Make sure after exactly 30 seconds (you can count using the clock, if it works), you hear the 'clunk' off your relay releasing. You can cross check this by looking at your dome light - it will visibly brighten when the glow plug relay releases. This is important to do first - if it is stuck, it will blow the fuse in your brand new meter since the glow plugs consume more than 10 amps, the max on most meters.

Okay, after you've done that:

- Test with the car off, key out of ignition.
- Remove negative battery cable from battery
- attach multimeter in series with negative battery cable, with meter set to measure 'current'.
- 4-5 ma is the amount I measure on my 300D with everything off
- note the current consumed. I had the same issue, and a current drain of 30mA was what I measured - enough to kill it overnight. In my case, it was traced to two of the cluster connections being mixed up by PO.
- pull one fuse at a time until you note when the current drops way down
- that will narrow it down to a system or two, look at the fuse map



Good luck :-) Let us know what fuse is causing you trouble...

dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car