how do I wire a pull switch light in bathroom?

i want to wire a pull swith light but when removing the old one there are two red wires connected to the exist old pull switch, the 2 blacks are not connected to the exist pull switch, just attached to a block & the earth is where it should be! Am i right in thinking that I can put one red in com, one red in either L1 or 2 & leave the blacks isolated?
Please advise/confirm.
Thanks Steve