If you know the wattage of all the lamps, then here's what to do. Sum the wattage, divide by ~10 to be conservative. This is the approximate amperage draw of all lamps (I have nto clue what the real formula with units is...I=PV or something like that...just that this is a rule of thumb).
Multiple this by 1.5 (150%) to arrive at a recommended peak amperage rating.
Then look at
this wire sizing chart to find the best wire to accomodate your wattage needs.
For example, let's say you have 6 65W lamps. That's 390W. Divide by 10, multiple by 1.5 (or divide by 6 2/3). You get 58.5 amps. According to chart, you should use 10ga wire. You can probably get by with 12ga wire, but 10ga is safer.
I'm not sure what stock wattage and wiring sizes are, but if you find the existing wiring is too small a guage, then you probably shoould wire a new harness on a separate relay just to be safe.