MerCruisers usually have an upper and lower shift cable. The lower shift cable is a bit tricky to install as it needs to be threaded thru the shift cable bellows and the bitter end installed. A service manual is a good idea.
The upper shift cable and throttle cables are more straight forward. Usually, the throttle cable needs to allow the throttle to stay closed when in neutral. The upper shift cable attaches to the shift assist switch, and then the lower shift cable.
If corrosion gets the lower shift cable, usually you have no reverse, as the cable cannot contract (it has been a long time, but I think that is correct)
You might check to see if you inadvertently switched the cables.