Is the relay working? But if you're putting 12 volts directly to the starter and it's not working, something else is up. I'd use a jumper cable to ground the starter to the negative terminal on the battery. I'd then use a jumper cable to provide 12 volts directly to the main terminal on the starter. I'd then use a small wire directly from the battery to activate the solenoid. If the starter does not run under those conditions either, the problem isn't in your wiring.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13