1987 W124 250D, European model, OM602 5-cyl engine.

My mechanic made the GP Relay fully manual, like an old W114/W115 /8.
He installed a switch under the dash to start the heating of the glow plugs manually. While he indicated to hold the switch for about 10 seconds, I found out that the actual holding length should be about 18 seconds on temperatures of +5 / -10 C. The car starts fine. Then alternatively, holding the switch in intervals of 2 to 3 seconds clears out all the smoking (if any). This is how I start my car, and so far it's been without problems. No more checking out if the GP Relay is bad or still good, no electronic-depending control, no more spending $132 on a new GP Relay.

This method also acts as a "theft deterrent". The car will not start at all if not activating the glow plugs via the switch. Only drawback is that in an emergency, the car will not quickly start as it has to follow the glow plug warming routine, but I believe that it's the same issue whether the glow plug control is manual or electronic. However, if the engine has just been shut off and it's been a couple of minutes after this, the car will still start without using the switch.

From your posts it seems that the actual glow plug activity is actually 30 seconds (not the 18 seconds that I count holding the switch at first start up). The actual activity time was something I wanted to know, so I thank you all for this information.