Are you trying to put in an additional halyard, or instrument wiring?
Hopefully, if you are attempting to install instrument wiring, there is a conduit or two built into the mast.
If they are there, and original to the boat, the builder of the mast has left a messenger line already there inside the conduit. Use it to pull in the new wire plus another messenger simultaneously. Then you will have a spare for next time. The mast on my boat has two such conduits, one all the way to the masthead, and the other ending at the steaming light.
Sometimes there is room in the conduit even with wire already in it to get a fish tape through it.
Another trick is to tie a piece of light line to a cotton ball, and use it to inhale the thing through the conduit with a shop vac, then pull through a messenger and your new wire.
As for a halyard, Shemander has the method in the post above