This is a very common problem to E38.
Do not trust your mechanic for that. 90% of wiper problem comes from the relay. Yes, the fuse may be good but this is relay problem. You need to check the relay unit. I can say this is one of the worse design of E38 which water leaks to the relay directly & cause rust & corrosion. This make the wiper not working.
At the top left corner of the engine compartment, there is where those relays are. If you have big hands, good luck than. The place you need to reach is very small, but it takes time to reach.
It won't cost you a lot . 50% is relay rust or defective, 50% just too much rust of the relay unit & you just need to deep ckean & scratch all rust out from the unit.