Well, I just viewed a wiring diagram and I was wrong about the safety switch wiring. Both front switches have their own ground circuits. This almost certainly means you have two separate problems.
It can not be a relay problem or your front windows wouldn't work (BTW the relay is behind the instrument cluster on a 123 body in a metal support that keeps it from being accessed from the bottom).
It cannot be a fuse as the windows are fused LF/RR RF/LR.
I would suspect switches first and stuck motors second. If you feel industrious the motors can be disassembled, cleaned and lubed and often work for years. A professional technician might not offer this service due to the risk of failure. We have done scores of them and see about a 10% failure rate within our one year warrantee period.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician