no speedo

hi all can anyone help me, I have pulled out all my hair now. I have a 1988 R20 2wd suburban, 454 ,th400 tbi. the Speedo stopped working sometime ago, 1st I replaced the cable, it wasn't broken, but I replaced it anyway, still no Speedo, 2nd I then replaced the Speedo head{ brand new gm genuine part still in the box}still no Speedo ,aaaaaaaaaaaaargh frustration now setting in, how unlucky can you get.3rd I then removed the broken speed gear, yippee yippee this must be it, but nope it wasn't, still no Speedo, next I removed the Speedo gear to inspect the gear on the drive shaft, now I am frustrated,its in perfect condition, still no Speedo. anyone got any ideas of how to fix this? p.s I put my drill on the Speedo cable and the Speedo works, but it doesn't work when connected to the gearbox. thanks in advance xxxxxl