I posted this on the mbz.org diesel list earlier today:

As many of you are probably aware, if you snip the violet wire at the GP relay, you can get your GP's to glow for an extended period after the engine is running. This should theoretically reduce smoke, emissions, and coking issues. The problem is that snipping the wire also forces the GP's to run 'afterglow' even when the engine is warm, which will diminish the life expectancy of the plugs. Many folks have wired in a manual switch to turn off this feature, but I wanted a more 'hands-free' operation.

I installed a thermally switched relay into the GP relay's violet wire circuit. I used an NC (normally closed) relay from a Volvo 700 series fusebox. These are a 5 terminal relay, similar in look to the familiar 4 terminal NO (normally open) versions used in Euro headlite conversions and countless other little projects. These NC relays have an additional 87a terminal to which one side of the violet wire is connected, opposite the 30 common for the other side. http://www.the12volt.com/relays/relays.asp Be careful of other similar 5 terminal relays with an 87b terminal--these are just NO relays with two bridged terminals--these won't work.

I piggy-backed onto the 50 C thermal switch above the thermostat housing on my '87 300 td OM 603, which is apparently designated to assist and smooth cold shifting in the transmission.

When the relay is energized, the violet wire is snipped--afterglow enabled. In this phase, the 50 C thermo-switch has continuity with ground.

When the relay is not energized, the violet wire is bridged--afterglow is disabled. In this phase the thermo-switch has no continuity with ground.

I'll let you know how well this works in real world situations.