hardest thing on electrical system ?

The biggest problems with old Mopars (and some others) in addition to being terribly "under" fused (protected) is the ammeter, and bulkhead connector. LOTS of these cars did "just fine" with no added accessories. But add "larger than" about a 60A alternator, and start adding stuff to the car, and you have trouble

The MAD article is a very good overview of the problem:

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

Also, headlight wiring was minimally small, and rear lighting harness was a bit light if you towed trailers, as I did with my 70 RR, which already has 4 stop/ tail lights

"Ma" knew this was a problem, as indicated if you can find the documentation on what is known as "fleet, police, taxi" wiring. This was used on 60? or 65A optional alternators, and basically, ran the ammeter wires through the firewall with individual grommets, bypassing the bulkhead connector.

So far as car washes, "I don't know." "Back then," I washed down the engine bay a LOT, on my 64, 69, and 70