I had a similar problem on my 2000 4Runner years ago where either of my taillights would randomly just not work. After multiple bulb replacements and re-seating and cleaning contacts that had no effect, I replaced the entire assembly and have never had problems since. Maybe it's a similar story for your Tacoma? It didn't sound like a common problem among 4Runners at the time. I actually upgraded to the '01-'02 4Runner taillights, which are much brighter than the '96-'00 4Runner taillights. Sorry I have no idea if there is a similar year of Tacomas with new brighter taillights that happens to fit in place of the crummier late-ninties taillights.