a different opinion
Marker Beacon's are going away, I wouldn't install a new one, but if you have one then as mentioned above almost any wire in the wingtip will work.
GS: You can put in your own dipole, or use a splitter on the VOR cable. If you have an SL-30 it has its own internal splitter, no option there. Splitters aren't cheap (like $100) but good coax isn't cheap either.
#2 Com: Unlike other reports, I am satisfied with my wing tip Archer-clone wing tip antenna for the #2 com. It is not as good as a "real" antenna, but not too bad, either. Ground stations regularly report it as "a little weaker than #1, but readable". I can get the ATIS above the static on #1 (external antenna) a few miles earlier than I can on #2 (wingtip). I have made no effort to swap radios to do a proper test. I will say that installation is important. I have a -10 so my wingtip is relatively thick. The leading edge of the antenna (where the feedline is attached) is placed at the thickest part of the wingtip, mounted as high as possible and then bent diagonally downward as much as possible. This is important as com's are supposed to use vertical polarization. Sometimes on the ground my belly whip is blocked by the airframe, and the wingtip works better! (but I view this as unimportant).