Not 100% sure. But pretty sure. Crimping is preferred in the factory because it's a simple mechanical process. If the crimp is closed with sufficient pressure, it will perform at least as well as solder. Doesn't really matter now that it's fixed.

"plastic piece on the striker"

Is this piece on the body, not on the door? If so, I'd think the problem is more in the mechanism on the door. The piece on the body, even when missing the plastic cover, should still lock the door.

If exposed to the elements, a crimped + soldered connection is superior. It will keep the copper strands from oxidizing/ corroding.

oh God I hope you're wrong, we'll know for sure tonight!!!

For your sake, I hope I am wrong also. If the new strikers don't fix the problem, Try squirting lot's of WD-40 in the door mechanism and blowing out the old crud with compressed air, keep doing that till no more crud come out then squirt some motor oil in there. If that does not fix it, take the mechanism out and see if you have any broken parts inside and do a better cleaning if nothing is broken.
I may have some good mechanisms if you need them.

I recall reading on this forum that if the charge indicator does not light during self test, the owners manual says to call MB roadside assistance immediately. What's the deal with that? An opportunity for the dealer to milk the customer?

I am going to take a look at my owners manual to see if it says that.

You'd think so, but no. There are a few problems with solder in high stress environments. The most important is that the mix of metals is prone to galvanic corrosion, so you may actually have more oxidation related issues than with bare wire. Worse, a solder connection can mask a bad crimp for a time (as in this case). Solder will work harden wherever it's providing primary mechanical support, and the result will eventually be a failure.

If you think about what you're doing, the connector that you solder to the end of a wire will either plug into a friction fit terminal or will be held down by a screw. In either case, you are dealing with a copper to copper (or brass) connection that relies on mechanical tension alone. If soldered connections are better for attaching a connector, why not just solder the wire directly to the terminal and dispense with the connector altogether? You'll almost never find a soldered connection in an OEM harness. Not 100%, but I would have guessed that someone was in here before.

The crimp terminal is tin plated, the stranded wire is bare copper, galvanic action right there from different metals. Solder will attach to both so there will be no more galvanic action. There is no stress from the solder, heat shrink is there to prevent any movement of the wire to crimp terminal.

Because connectors is a necessary evil. Would you rather unplug a connector or un-solder 30 or more wires to service an ECU?

There are many soldered connections in W123's. Any PC boards with electronic components are soldered, All the connector terminals on plugs and sockets are soldered to the wires. I have not seen any of those wire terminal soldered connections fail. All the failures I have seen are corrosion in the male to female connector contacts. So yes, solder connections are better, as long as they are stress relieved where the wire is not allowed to move/ vibrate where the wire to solder joint is.

Tin is a component of lead based solder and isn't considered a dissimilar metal relative to say aluminum and steel. Melt copper and tin and you have the alloy bronze.

Really good copper wire is tin plated for corrosion resistance.

A properly crimped joint will cold weld resulting in a gas tight seal that will keep corrosion out of the joint. If a cold weld joint isn't produced, corrosion can work it's way between the strands and terminal.

The problem with soldering is solder wicks up the strands causing a stiff area away from the terminal that is susceptible to fatigue cracking.

Still waiting for the UPS truck to show up... lol

Replaced all four strikers. The doors close. Easier than ever before, but not like new cars; you need to give it a firm 'follow-through' for them to close.

Good it worked! Have you done the charging tests?