I'll not CONDEMN any electrical appliance (Much less order one for $Dollars$)
UNTIL I have proof positive from EXHAUSTIVE DMM [Digital Multi-Meter]Testing
results!
(Without Empirical testing results,You can "Throw" Hundreds+ of Dollars U.S.
at this chassis and NOT FIX ANYTHING!)
All that you KNOW,right know, is that you've some kind of Electrical Anomaly
[Electrical Difficulties that do not follow the paths called wiring have three names: 1. Lightning 2. EMP Pulse 3. Divine Intervention.]
FSM page OnLine that deals with Pre-Glow Relay Testing:
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/Engine/602_603/15-4000.pdf
You want to utilize SECTION "B" because your Relay has "AfterGlow".
Before I'd BUY a new one
(Even if the old one was sticking...
[SEE JEREMY's post on AfterGlow indicator light:
http://www.peachparts.com/shopforum/showthread.php?t=236848&highlight=afterglow]
AND I could "SEE" the ReLay sticking...]
Take the relay apart GENTLY and clean the contactors and apply Stabilant 22
to both the High and Low Amp contactor (s).
AND you're correct, the RED wire feeds battery Voltage and Amperage to
the ReLay, At All Times. (No 80 Amp Fuse).
Electrical problems are much like Pyramids , they must have a stabile/firm
base to build upon (No Shortcuts).
Sometimes you have to Start from ZERO (Grounds) and work your way up!
I see that you've been a Fomoco customer in days of yore, so you've probably a much nicer DMM than I do.
BUT if not ...Sears and Harbor freight regularly have sale on them for less than $20.