Generally, located on the front of the driver's fenderwell, but you should be able to follow the wiring harness back from the glow plugs to the relay.

First, when you find the relay, there's a wire that goes to the dash light. Apply +12V to pin #3 on the four-pin plug and make sure the light works. It probably does. You may have only a single glow plug out. The relay module has a comparator circuit in it that measures the #1 glow plug against the sum of the rest of the plugs. Check #1 first, and if it isn't right, replace it and see if the light comes on then. If it does then save the rest of the glow plugs for issues further down the road. If it doesn't, replace them all. They're probably due.

Also remove the fuse, inspect it for a crack in the middle (it happens) and re-install it if it looks good. Keep the strip fuse in the glove box just in case.

My regards to your father for his service.

__________________
mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)