2005 Hyundai H100 Gearbox problem

My 2005 Hyundai H100 has a few problems. 3rd and 5th gear won't engage - had the gearbox replaced - more than 400 000km's - replaced the gear selector cable and gear-shift "ball" in cab, but to no avail - 3rd will sometimes engage, but 5th not at all.
Another problem is fuses for indicators and hazard lights keep going - within half an hour of replacement.
Any advice/ideas?
Thank you!