EDIT: Just so all of this won't be "All Fun,All the Time"...
Physically locate the separate "Ground" wire for the Electronic Pre-Glow Relay
at it's terminus on the body.
And assure yourself the Connections and the Cable are good.
An intermittent Grounding Connection sounds awfully like your symptoms, TOO
(I admit I'm grabbing at non-existent Life Rings, here.)
BUT, maybe, each time you R+R the Relay to clean the contacts / you also
"Fix" a Ground problem,Temporarily.
1992 is awfully close to the "Middle Nineties"
"Automatic Self (Ecologically) Degrading Engine Wiring Harnesses"
Mercedes bought from a Vendor and "Gifted" [Spelled, F**ked] some owners with.
(Recycling was all the rage in Deutschland, and the Vendor screwed up on the insulation specifications. OR MAYBE NOT?!?)
I'd clean the contactor(s) once more and make sure they're super duper SHINY and Smooth/Mirror Like and then apply Stabilant22 to both sides.
(It's about $30.00 for 5Ml which makes 15Ml solution)
[OUTRAGEOUS! But the stuff works.]
And it's cheaper than a New GP Relay.
http://www.stabilant.com/techt02h.htm
http://www.stabilant.com/sizes01h.htm
Example of pricing:
http://www.posthorn.com/Stab_2.html