just did this on my 77 w200 yesterday.
there has been an aftermarket ammeter in the truck since i got it. i drove it all morning yesterday running errands and i smelled some hot plastic, also noticed the ammeter was showing about 40amps charge when the rpm's were high enough for the alternator to put that out.
i had previously pulled the bulkhead connectors off and sprayed some deoxit in there and worked the connectors in there a few times in hopes of buying some more time.

thought about what i was going to do, decided to pull that connector where the black wire from the alternator goes into the bulkhead, it was melted pretty good. i ended up taking a larger wire from the alt output to the batt+ on the relay, also ran a 10g wire from that same post and went through a hole in the firewall right below the bulkheads. i cut the wire back where it feeds to the wiring harness for power and to the ammeter inside the cab, some of it was burned and corroded. just used a butt connector to attach my 10g wire to the original black 10g wire. so bypassed the bulkhead where it melted.
i thought i had a voltmeter hanging around to replace the ammeter but didn't so just left the ammeter. it's not accurate now but i don't really care. just needed to get the truck running again.

i think at this point i might as well put one of the denso alternators on it and put a voltmeter in it....