GP Controler ?
 

My 96 won't start when the temp is below 40.
My indi replaced the glow plugs-2 were bad.
But it still won't always start.
GP light doesn't always come on.
Tech thinks it is the GP controller.
What is that?

Pre-Glow Relay
 

Part # (MAY BE) 0155453032 ($140-150 'ish)

Check your Glow pugs again , If one or more is bad the dash light may not
illuminate to let you know of a problem (usually the#1 plug)

See if there is a strip fuse inside the relay itself (It must be tested with a
multi-meter for continuity,visual inspection may miss a hairline crack)

Check the wiring diagram for a fuse in the fuse box that protects the dash
"Pre-Glow relay indicator light".

Is it throwing any codes?

There are six small fuses inside the unit. I don't think they are intended to be replaced.

Pretty easy to test with a multi meter.

Go to dieselgiant.com and follow the tutorial there for the W123.

Dash "Glow Light"bulb out?
 

I know , It's Too Easy.

But,remember you may have two(2) problems with the Glow system,and the
Light on the dash Might be a separate issue from the unhappy start.

Multimeter with back light on sale at Harbor Freight $4.

Thats the one I bought; I'm electrically challenged and figured that tools quality was greater than my ability.;)