It's way past midnight, but he doesn't care: The slovenly, overweight sad sack with nothing to live for barrels down the interstate in his souped-up pickup zonked out of his mind, blaring rock & roll and aiming to plow down anything and everything in his way. That's the milieu conjured up by the inspired manic roar of Tad, a Seattle grunge band led by slovenly, overweight singer-guitarist Tad Doyle. The characters on 8-Way Santa, the band's third and most coherent album, drive drunkenly into lakes, proudly wear smelly clothes, and mutter things like ''So messed up you can't believe!'' and ''No one touches my kids/Except me!'' Tad doesn't relent on the music, either. Santa is biker rock driven by crushing slabs of guitar potent * enough to kill baby seals at 50 paces. The band pauses for a few airy guitar chords and tosses off a downright poplike chorus or two, but Tad's music is best summed up by one of its lyrics: ''Better not get in my way/Better hide behind your door.'' No problem, dude.