I wish Mercedes gives lifetime warranty here. As far as I know only FCP Euro does that for us.
I've been using PIAA replacement bulbs. Japanese company, supposedly super output, in the fog lamp position. When I change a single burned out one they are significantly brighter than the old ones. I went to Candlepowerforums to find they are junk, but had been happy with them all this time, paying extra for what I perceived was good quality stuff. With all honesty, smooth is exactly why I'm using the plugs I do, as you non-resistor, because the non-resistor type make your car run smooth. I'm confident I don't need to worry about them wearing down in 15k or 30k miles. I need to look somewhere else if there's a breakdown. That's pretty much the difference.
Anyways, regarding relays, from what I learned from school is that they are what is called "solid state", meaning no real moving parts. They are hard to break because of this. Relays are basically switches, and "supposed" to be very reliable. Haven't had to replace a fuel pump relay yet, but for now the W126 is not being used. The cheap ones don't work? I'm really not familiar with the situation... I would expect decent relays from most places. I bought the cheapest one I could find when the repair shop said the problem was with it, for the AC system in my W123. Turned out it was severe water damage to the "EGR" due to poor windshield replacement seal, and they used a simpler relay for the repair, connecting only a few pins on the harness connector. (They either weren't being honest, or didn't catch the exact issue until later). Beyond that, only window switches, window regulators, and tune ups: wires, cap, rotor, and plugs, have solved most of my problems I can think of on the top of my head, electrically, that I've personally repaired, that is. (Not sure what happens when I take the car somewhere to get repaired. I have gotten a bill, after requesting all old parts back, and there are items there and no old part!)
Sorry if I'm being trouble.
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