You haven't provided the s/n of your engine but here is a link to the LU of a 1986 115. Are you sure it is a wire that you are seeing or could it be item 2, which is a hose for the speedometer? It just connects to another hose running forward to a speedometer, if you have one installed.
Mercury, Mercruiser, Force, etc. - GEAR HOUSING (DRIVE SHAFT) - 90/115 H.P. (INLINE)
Gerry