I recommend beefing up grounds on * any *
electrical system. This goes double for modern
cars with computer controlled devices.
The cost is low, start by running a heavy jumper from
engine block to frame. A Ford starter cable works well.
Use stainless steel screws / bolts whenever possible, paint over connections
to seal out moisture. A jumper from frame rail to radiator support, and
another to cowl will
pay off over the years. Run a long jumper from known good ground and probe
ground screw for tail light harness. If bulbs glow brighter, run a jumper
from ground terminal to frame rail. Cherck at night, if bulbs barely glow,
that sounds like ground to me !
Stop lights often have their own " floating " ground
for safety reasons. Headlights go one step beyond, and are not only
isolated, but have a circut breaker too !