is there any scematic or such to help bench test a glow plug timer or relay with a multi meter for continuity and such?

my glow plug light stopped coming on a few days ago and the immediate result is the car went from super easy cold start to damn near impossible to start....

I have the Euro Haynes book and all it says is replace glow plugs, temp sensor or relay.

the temp guage works fine so I'm guessing that is the same temp sensor that sends a signal to the relay to emergize glow plugs if it's too cold so that is likely not the problem.

I'm guessing it's the relay or power to the relay...

If the glow plug light came on and it won't start that would indicate several or all glow plugs are bad.

There is no fuse that I have found that feeds the relay...but i'm sure there is one...it's just not listed in the repair manual and not labeled on the fuse box...

any help or suggestions to further trouble shot the relay or find a fuse before I spend $100+ on a new relay I may not need?

__________________
1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5