I used a Painless wiring kit on my CR Cobra and, like DV says, I put a bunch of chassis grounds all over the place. Grind the paint off the chassis, drill and tap a hole, use a #10 screw (or larger) with a star and flat washer; cover the screw and ring lug with dielectric grease after everything is installed. And yes, I did install a separate "extra" ground for the tank sender
Lights: check the voltage at the light terminals with the lights on. If you've got a bad connection or too small of wire you'll see a voltage drop at the load (the lights in this case). If you want to get real fancy you can then check the voltage from the positive light terminal to the positive terminal on the batter to determine the line drop and from the light terminal ground to battery ground to determine the ground line drop.