I'm having issues with my wipers and think it's a 1987 mercury grand marquis wiper switch? How do I know if it's the actual switch that's the issue?

Try carefully taking the switch apart, after disconnecting the negative battery cable. Look for dirt, grease, and wear that may be causing a problem. Depending on the switch, try checking for continuity. If the problem persists, you will likely be forced to replace it. You may want to try your local salvage yard.