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 !