wiper motor not working

[67 swb]

Need help figuring out whats going on.. Working on my 68 gmc wipers are the only thing I haven't been able to get working correctly.. I have 12v on yellow wire at wiper motor.. Wiper switch switches ground.. I can run a jumper wire to ground anywhere on truck to black wire on wiper switch and wipers will work.. What I'm confused about is I can do a continuity test with meter on black wire at plug and I will get continuity on any ground point in the truck.. What am I missing.. Any ideas would be appreciated..