The diagram on my CD-ROM, P-107, PREGLOW SYSTEM (as of MY 1986) shows a 'Run-Start Input' (Terminal 1) and 'Cranking Input' (Terminal 2) to the relay.

These are noted as pins on an X2 four-pole connector, with Terminal 3 to the Glow Lamp, and Terminal 4 is a controls ground.

So, the GP relay appears to get separate RUN and Cranking signals. The M-B engineers do not elaborate what is inside the 'black box' but there could well be internal logic that keeps the GPs energized during cranking and drops the relay when cranking stops.

This could be easy to test with a voltmeter during cranking.

The other connections to the relay are:

A Red wire (Terminal 30) supplies power at all times, which is routed through a fuse and relay contact to the individual GP leads via the X1 6-pole connector.

A single white wire supplies a signal from a coolant temperature switch. but this also appears to go to the glow lamp indicator.

There is noted an internal air temperature resistance function.