Guys;

Lockwashers, whether internal or external, loose or captured are always a good idea for biting into the metal and making a good electrical connection between a crimp terminal and chassis ...and combined with a good precleaning/abrading and ACZP to keep it clean and uncorroded, the connection can be trusted for pretty much ever!

Hint: Don't forget to inspect crimp for oxidation, damaged conductor strands, and/or vibration damage! Wires themselves rarely go bad...terminations fail ten times more often!

I disagree with info from other poster...according to the wiring diagram, chassis grounds are SEPARATE and made at EACH lighting fixture, so a failure of BOTH tail lights suggests a (single) supply problem, not (two) open chassis connections...besides, other lamps which use those chassis connections are functional.

Good Hunting!