Power door locks UNlock but won't Lock on '95 GXE

I have a '95 Sentra GXE that unlocks with gusto, but will not lock, except manually. It has a manual transmission and no alarm. It is stock. It has not been wet, and has gotten very hot in the sun all this week with no change. I can operate the unlock from either front door switch, but neither will lock anything. When I disconnect and reconnect the SECU the doors ALL INSTANTLY lock. When I remove the SECU and try others (four so far, two from identical cars and two from other years), I get the same results. So, I know the wires to and from all the doors work. What am I missing? Everyone else seems to have the opposite or an intermittent problem. The only other problem that I have is the wipers don't work on the "UP" stroke on the delay setting, otherwise they are fine. (I can hear the relay on the delay setting for the wipers, but not for the door lock function.) How do I troubleshoot this further and are the two in any way related?

Thanks for your help!