Why is passenger side head light dim

If the bulb is still dim when using another one, likely it is a faulty connection. A poor ground or power connection anywhere along the harness could cause a voltage drop.
If you don't have one, you can get a small tester like that from Harbor Freight, etc. for less than 10 bucks and use it for checking the line from the bulb all the way back. Check the other side and see how much voltage is going to the bulb that is working correctly as a reference.

It could be a bad headlight switch, a bad connection at the headlight or a bad ground. Make sure all connections are free of corrosion. Also make sure no after market items have been added improperly wired. Like fog lights trailer plugs etc.