79 south to exit 141 (interchange RT 285) RT 285 West (less than 1/8 mi.) to RT 19 south. Follow RT 19 south to Georgetown Rd. (there will be a restaurant on the left (County Line Cafe)). Turn left onto Georgetown Rd. which will take you straight to Lake Wilhelm. HOWEVER, you will encounter a detour do to construction on Georgetown Rd but, not to worry the detour is well marked. There is a bait/tackle shop at the Marina and another place called Fergies on the opposite side of the lake. Take the causeway across the lake and turn left at the stop sign ( this will be Georgetown and you will encounter the other end of the detour at this point) Fergies will be on your right about 1/4 mile after turning left.
Last I knew the detour was still in effect. If the detour is down Fergies will be on your left at Vacation Campground entrance and the causeway will be on the right about 1/4 mi.
There is a second way but, I think even with the detour, you'll get there faster.
If I might be of further assistants shoot me a PM.