Yep, remove and bench test.
Your description of them acting like they are leaking together is a ground problem. The bulbs basically end up in series and run dim.
You have more than one problem so sort out each ground issue.
One way is to make your own ground wire as a temporary tester. Make it plenty long and find a shiny place to connect it. Then go to one of the lights that is acting dimly and figure out which side should be ground by taking the bulb out and using a voltmeter to test. Then ground the proper side while testing. Use a voltmeter and your handy new ground for the negative lead of the voltmeter. Test the hot side of the bulbs.
