You need to do some testing , and lucky for you all it takes is a cheap volt meter from Autozone or the shack .
Test the volts at the light , rev the engine some , now test the volts at the battery , are they close ? Another way to test the circuit is called the volt drop method . This will tell you the dynamic resistance of a working circuit . With the light on put one lead on the battery post and the other on the light positive , the reading on the meter will be your volt drop . You might see .05 volt on your brand new digital meter . If you see .2 or something of that nature , you have resistance somewhere in the circuit . You can also test the ground side , and this is the one I suspect if both beams are dim . Simply put one lead ( dosen't matter which in either example) on the negative terminal of the battery , and the other on the ground lead , again about the same voltages apply . If you are having some performance issues you may actually have a system that's running too low on voltage , I'd guess it needs to be 13 or better , my beemer lives with that when I have the extra lights turned on .