20-22 is too small for that, IMO. Use 18 ga.
The reason your center lights are dimmer is because you are feeding off of a ground that goes through the filament of the corner light. The filament decreases the strength of the ground/negative charge so your light isn't as bright.
If it still doesn't make sense I will see if I can draw a picture explaining it a little better.
BTW, if you hook up the ground to the center lights and still keep the wire connected to the corner lights you will blow fuses when you turn your signal lights on because you are creating a short. So either leave the ground for the center lights hooked to the corner light for the alternatively flashing lights, or hook the ground for the center lights backup, but remove the wire going to the corner lights. (You of course will not have alternating lights if you do the latter).