It's very unlikely the clearance lights would be wired in series. Each bulb would have to be rated for about 4 volts, if there are 3 lights in the center of the cab, or of there are 5 lights like the 18 wheelers run, each bulb would have to work on about 2 1/2 volts each. Also, one burned out bulb in a series circuit would kill all the lights. You can run one wire all the way across, and splice individual lights into the single supply wire. It will still be a parallel circuit, since each light will have its own ground to the cab metal.
Jerry
