crutchfield says that the (98) 4runner has 6 3/4", wierd size. I bet if you wanted to, you could fit a 7" in there. Also many companies do make 6 3/4". Also look what a found! I bet you it maked the system sound much better, I wonder if its difficult adding to more spaekers to your system?
http://www.crutchfield.com/cgi-bin/S-0kE0tJR2la1/ProdView.asp?s=0&c=10&g=765&I=044K400387&o=m&a=1&c c=01&avf=Y