Can't picture the horn assembly in my head right now, but if you are not reusing the horn you could probably just destroy that to get to it. Otherwise you can get one of the drill bits that pull out stripped screws. I have never had much luck with them, but maybe you will.
Sold:2004 XLT 4x2 Ext. Cab. 3.0 Mods: 3" spindle, AAL, shackle, 32x11.5 on 15x8s, billet grille, clear corners, flow 40 dumped, line-x, CB/PA, traction bars, lil sound system, auto up down windows
Sold:2005 XLT 4x4 Ext. Cab. 4.0 Mods: ProComp lights w/ HID swap on tabs, 32x11.5 on 15x8s, CB/PA, black out heads/corners, lil sound system, cranked t-bars, flow 40 dumped, line-x, shell, bed rug, 06+ tails, metal tail gate handle since plastic one shattered, OHC, Mustang Dome.